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A    LABORATORY   MANUAL  AM)   TEXT- BOOK. 


of 


EMBRYOLOGY 


By 

CHARLES  WILLIAM  PRENTISS,  A.M.,  Ph.D. 

PROFESSOR  OF  MICROSCOPIC  ANATOMY  IN  THE  NORTHWESTERN  UNIVERSITY  MEDICAL  SCHOOL,  CHICAGO 


WITH    368    ILLUSTRATIONS 
MANY  OF  THEM   IN  COLORS 


PHILADELPH]  \    \\!>  LONDON 

W.  B.  SAUNDERS  COMPANY 

1915 


Copyright,    iqis   by  W.  B.  Saunders  Company 


PRINTED     IN     AMEHICA 

PRE8S    OF 
W.    B.    8AUN0ER6    COMPANV 

PHILADtLPHIA 


PREFACE 


This  book  represents  an  attempt  to  combine  brief  descriptions  of  the  verte- 
brate embryos  which  are  studied  in  the  laboratory  with  an  account  of  human 
embryology  adapted  especially  to  the  medical  student.  Professor  Charles  Sedg- 
wick Minot,  in  his  laboratory  textbook  of  embryology,  has  called  attention  to  the 
value  of  dissections  in  studying  mammalian  embryos  and  asserts  that  "  dissection 
should  be  more  extensively  practised  than  is  at  present  usual  in  embryological 

work "     The  writer  has  for  several  years  experimented  with 

methods  of  dissecting  pig  embryos,  and  his  results  form  a  part  of  this  book.  The 
value  of  pig  embryos  for  laboratory  study  was  first  emphasized  by  Professor  Minot, 
and  the  development  of  my  dissecting  methods  was  made  possible  through  the 
reconstructions  of  his  former  students,  Dr.  F.  T.  Lewis  and  Dr.  F.  W.  Thyng. 

The  chapters  on  human  organogenesis  were  partly  based  on  Keibel  and 
Mall's  Human  Embryology.  We  wish  to  acknowledge  the  courtesy  of  the  pub- 
Ushers  of  Kollmann's  Handatlas,  Marshall's  Embryology,  Lewis-Stohr's  Histology 
and  McMurrich's  Development  of  the  Human  Body,  by  whom  permission  was 
granted  us  to  use  cuts  and  figures  from  these  texts.  We  are  also  indebted  to 
Professor  J.  C.  Heisler  for  permission  to  use  cuts  from  his  Embryology,  and  to 
Dr.  J.  B.  De  Lee  for  several  figures  taken  from  his  'Principles  and  Practice  of 
Obstetrics."  The  original  figures  of  chick,  pig  and  human  embryos  are  from 
preparations  in  the  collection  of  the  anatomical  laboratory  of  the  Northwestern 
University  Medical  School.  My  thanks  are  due  to  Dr.  H.  C.  Tracy  for  the  loan 
of  valuable  human  material,  and  also  to  Mr.  K.  L.  Vehe  for  several  reconstruc- 
tions and  drawings. 

C.  W.  Prentiss. 

Northwestern  University  Medical  School, 

Chicago,  III.,  January.  1915. 


CONTENTS 


PAGE 

Introduction 1 1 

Chapter  I      I'm:  Germ  Cells 17 

The  ( )vum ■ 17 

Ovulation  and  Menstruation 20 

1'he  Spermatozoon .1 

Mitosis  and  Amitosis 22 

Maturation J4 

Fertilization 30 

The  Chromosomes  in  Heredity 

The  Determination  of  Sex ji 

Chapter  II. — Segmentation  and  Formation  of  the  Germ  Layers 3$ 

Segmentation  in  Amphioxus,  Amphibian,  Bird,  and  Reptile $$ 

Segmentation  of  the  Rabbit's  Ovum 36 

( )rigin  of  the  Ectoderm  and  Entoderm 37 

Origin  of  the  Mesoderm,  Notochord  and  Neural  Tube 42 

Origin  of  the  Mesoderm  in  Mammals 45 

The  Notochord 41 , 

Chapter  III. — The  Study  of  Chick    Embryos 48 

Chick  Embryo  of  Twenty-five  Hours 48 

Transverse  Sections  of  Twenty-five-hour  Chick  Embryo 50 

Origin  of  the  Primitive  Heart 52 

Chick  Embryo  of  Thirty-six  Hours  ( 18  segments) 54 

Central  Nervous  System 54 

Digestive  Tube 55 

Heart  and  Vessels '56 

Mesodermal  Segments 61 

Ccelom 63 

Mesenchyma 63 

Derivatives  of  the  Germ  Layers 64 

Chick  Embryo  of  Fifty  Hours  (27    segments) 65 

Nervous  System 66 

Digestive  Organs 67 

Blood  System 67 

Study  of  Transverse  Sections 69 

Amnion,  Chorion 74 

Yolk-sac  and  Allantois 75 

Chapter  IV. — The  Fetal  Membranes  and  Early  Human  Embryos 77 

Early  Human  Embryos 77 

Fetal  Membranes  of  Pig  Embryos 7  7 

Umbilical  Cord 79 

Fetal  Membranes  of  Early  Human  Embryos 80 

Chorion 81 

Amnion s  ^ 

Allantois N  | 

Yolk-sac  and  Stalk 86 

Anatomy  of  a  4.2  mm.  Human  Embryo 88 

Central  Nervous  System 80 

Digestive  Canal 80 

Urogenital  and  Circulatory  Organs 02 

Age  of  Human  Embryos 95 

Chapter  V. — The  Study  of  Pig  Embryos Q7 

The  Anatomy  of  a  six  mm.  Pig  Embryo 07 

External  Form 07 

Internal  Anatomy 00 

The  Study  of  Transverse  Sections 1 1 1 


8  CONTENTS 

PAGE 

The  Anatomy  of  a  10  mm.  Pig  Embryo 120 

External  Eorm 1 20 

Central  Xervous  System  and  Viscera 122 

Heart  and  Blood  Vessels 128 

The  Study  of  Transverse  Sections 132 

Chapter  VI. — Methods  of  Dissecting  Pig  Embryos:  Development  of  the  Face,  Palate, 

Tongue,  Teeth  and  Salivary  Glands 145 

Directions  for  Dissecting  Pig  Embryos 145 

Lateral  Dissections  of  6-35  mm.  Embryos 146 

Median  Sagittal  Dissections 148 

Ventral  Dissections 153 

Development  of  the  Face  in  Pig  and  Human  Embryos 153 

Development  of  the  Hard  Palate 155 

Development  of  the  Tongue 158 

Development  of  the  Salivary  Glands 161 

Development  of  the  Teeth 162 

Chapter  \TI. — Entodermal  Canal  and  its  Derivatives 168 

The  Pharyngeal  and  Cloacal  Membranes 168 

Pharyngeal  Pouches  and  their  Derivatives 168 

The  Thymus  Gland 171 

The  Epithelial  Bodies  or  Parathyreoids 172 

The  Thyreoid  Gland 172 

Larynx.  Trachea  and  Lungs 173 

Digestive  Canal 177 

Digestive  Glands :  Liver 183 

Pancreas 186 

Body  Cavities,  Diaphragm  and  Mesenteries 188 

Primitive  Ccelom  and  Mesenteries 188 

Septum  Transversum 191 

Pleuro-pericardial  and  Pleuro-peritoneal  Membranes 192 

Diaphragm  and  Pericardial  Membrane 195 

Omental  Bursa,  or  Lesser  Peritoneal  Sac 197 

The  Mesenteries 200 

Chapter  VIII. — Urogenital  System I 203 

Pronephros 203 

Mesonephros 205 

Metanephros .- 207 

Nephrogenic  Tissue 21c 

Cloaca,  Bladder,  Urethra  and  Urogenital  Sinus 213 

Genital  Glands  and  Ducts 216 

Testis 218 

Ovary 221 

Union  of  Genital  Glands  and  Mesonephric  Tubules 225 

Uterine  Tubes,  Uterus  and  Vagina 226 

Ligaments  of  the  Internal  Genitalia 228 

Descent  of  Testis  and  Ovary 230 

External  Genitalia 232 

Male  and  Female  Genitalia  Homologizcd 235 

The  Uterus  during  Menstruation  and  Pregnancy 238 

The  1  >eciflual   Membranes 239 

Chorion  Laeve  and  Frondosum 243 

Decidua  Vera 243 

Do  idua  Capsularis 244 

The  Placenta 245 

The  Relation  of  Fetus  to  Placenta 249 

Chapter  K. — Vascular  System 251 

The  Primitive  Blood-vessels  and  Blood-cells 251 

Erythrocytes 252 

Leu<  01  ytes 253 

Blood  plates 254 

The  I  development  of  the  Heart 255 

Primitive  Blood  Vascular  System 267 

I  development  and  Transformation  of  the  Aortic  Arches 270 

Vertebral  and  Basilar  Arteries 272 

Segmental  Arteries 274 

Umbilical  and  Iliac  Arteries 275 


CONTENTS  g 

PAOI 

Arteries  of  i In-  Extremities zj$ 

Vitelline  and  Umbilical  Veins:   Vena  Porta 

Anterior  ( Cardinal  Veins:  Superior  Vena  ( lava .   279 

Posterior  Cardinal  Veins:   Inferior  Vena  Cava _\So 

The  Veins  of  the  Extremities ->s^ 

The  Petal  ( Circulation 

The  Lymphatic  System 286 

Lymph  Glands 287 

Ikemolvmph  Glands  and  Spleen .    288 

Chapteb  X.    Histogenesis 290 


The  Entoderms!  Derivatives 

Tile  Mesodermal  Tissues. 


290 
291 


Si  lerotomes  and  Mesenchyme 29 

Supporting  Tissues 29 

Cartilage .  294 

Bone 205 

Joints 298 

Muscle 298 

The  Ectodermal  Derivatives 303 

Epidermis '^04 

Hair 3o5 

Sweat  Glands 307 

Mammary  Glands 307 

Nails 308 

The  Nervous  Tissues 309 

The  Differentiation  of  the  Neural  Tube 310 

Neurones  of  the  Ventral  Roots 313 

Spinal  Ganglia  and  their  Neurones .  313 

The  Neurone  Theory .  315 

The  Supporting  Tissue  of  the  Nervous  System 316 

Chapter  XI. — Morphogenesis  of  the  Central  Nervous  System 31Q 

The  Spinal  Cord 320 

The  Brain 325 

The  Differentiation  of  the  Subdivisions  of  the  Brain 330 

Myelencephalon .  330 

Metenccphalon 333 

Cerebellum 334 

Mesencephalon ^^^ 

Diencephalon 336 

Hypophysis 337 

Telencephalon 33g 

Chorioid  Plexus  of  Lateral  Ventricles 340 

Cerebral  Hemispheres 346 

Chapter  XII. — The  Peripheral  Nervous  System ^  t 

The  Spinal  Nerves •  - 1 

Brachial  and  Lumbo-sacral  Plexuses 353 

Cerebral  Nerves .  355 

Special  Somatic  Sensory  Nerves 335 

Somatic  Motor  Nerves 338 

Visceral  Mixed  Nerves 35g 

The  Sympathetic  Nervous  System .  364 

Chromaffin  Bodies:    Suprarenal  Gland .  366 

The  Sense  Organs .  368 

General  Sensory  Organs .  368 

Taste  Buds .  368 

Olfactory  Organ .  369 


Eve. 


373 


Ear 381 

Index 389 


TEXT-BOOK  OF  EMBRYOLOGY 


INTRODUCTION 

The  study  of  human  embryology  deals  with  the  development  of  the  individual 
from  the  origin  of  the  germ-cells  to  the  adult  condition.  To  the  medical  student 
human  embryology  is  of  primary  importance  because  it  affords  a  comprehensive 
understanding  of  gross  anatomy.  It  is  on  this  account  that  only  recently  a 
prominent  surgeon  has  recommended  a  thorough  study  of  embryology  as  one  of 
the  foundation  stones  of  surgical  training.  Embryology  not  only  throws  light 
on  the  normal  anatomy  of  the  adult,  but  it  also  explains  the  occurrence  of  many 
anomalies,  and  the  origin  of  certain  pathological  changes  in  the  tissues.  From 
the  theoretical  side,  embryology  is  the  key  with  which  we  may  unlock  the  secrets 
of  heredity,  of  the  determination  of  sex  and,  in  part,  of  organic  evolution. 

There  is  unfortunately  a  view  current  among  graduates  in  medicine  that  the 
field  of  embryology  has  been  fully  reaped  and  gleaned  of  its  harvest.  On  the 
contrary,  much  productive  ground  is  as  yet  unworked,  and  all  well- preserved. 
human  embryos  are  of  value  to  the  investigator.  An  institute  of  embryology 
for  the  purpose  of  collecting,  preserving  and  studying  human  embryos  has  re- 
cently been  established  by  Professor  F.  P.  Mall  of  the  Johns  Hopkins  Medical 
School.  Aborted  embryos  and  those  obtained  by  operation  in  case  of  either  normal 
or  ectopic  pregnancies  should  always  be  saved  and  preserved  by  immersing  them  intact 
in  10  per  cent,  formalin  or  Zenker's  fluid. 

The  science  of  embryology  is  a  comparatively  new  one,  originating  with  the 
use  of  the  compound  microscope  and  developing  with  the  improvement  of  micro- 
scopical technique.  Chick  embryos  had  been  studied  by  Malpighi  and  Harvey 
previous  to  Leeuwenhoek's  report  of  the  discovery  of  the  spermatozoon  by 
Dr.  Ham  in  1677.  At  this  period  it  was  believed  that  the  spermatozoa  were  both 
male  and  female  and  developed  in  the  ovum  of  the  mother;  that  the  various  part s 
of  the  adult  body  were  preformed  in  the  sperm-cell.  Dalenpatius  (,1699)  believed 
that  he  had  observed  a  minute  human  form  in  the  spermatozoon.  Previous  to 
this  period,  many  animals  were  believed  to  be  spontaneously  generated  from  slime 
and  decaying  matter  as  asserted  by  Aristotle.  The  preformation  theory  was  first 
combated  by  Wolff  (1759)  who  saw  that  the  early  chick  embryo  was  differentiated 


1 2  INTRODUCTION 

from  unformed  living  substance.  Tins  theory,  known  as  epigenesis,  was  proved 
correct  when,  in  1827,  von  Baer  discovered  the  mammalian  ovum  and  later 
demonstrated  the  germ-layers  of  the  chick  embryo.  When,  after  the  work  of 
Schwann  and  Schleiden  (1839),  the  cell  was  recognized  as  the  structural  unit  of 
the  organism,  the  ovum  was  regarded  as  a  typical  cell  and,  in  1843,  Barry  ob- 
served the  fertilization  of  the  rabbit's  ovum  by  the  spermatozoon.  Hence- 
forth all  multicellular  organisms  were  believed  to  develop  each  from  a  single 
fertilized  ovum,  which  by  continued  cell-division  eventually  gives  rise  to  the 
adult  body.  In  the  case  of  vertebrates,  the  segmenting  ovum  differentiates  first 
three  primary  germ-layers.  The  cells  of  these  layers  are  modified  in  turn  to  form 
tissues,  such  as  muscle  and  nerve,  of  which  the  various  organs  are  composed,  and 
the  organs  together  constitute  the  organism,  or  adult  body. 

Primitive  Segments — Metamerism. — In  studying  vertebrate  embryos  we 
shall  identify  and  constantly  refer  to  the  primitive  segments  or  metameres. 
These  segments  are  homologous  to  the  serial  divisions  of  an  adult  earth- 
worm's body,  divisions  which  are  identical  in  structure,  each  containing  a 
ganglion  of  the  nerve  cord,  a  muscle  segment,  or  myotome  and  pairs  of  blood- 
vessels and  nerves.  In  vertebrate  embryos  the  primitive  segments  are  known 
as  mesodermal  segments,  or  somites.  Each  pair  gives  rise  to  a  vertebra,  to  a  pair  of 
myotomes,  or  muscle  segments,  and  to  paired  vessels;  each  pair  of  mesodermal 
segments  is  supplied  by  a  pair  of  spinal  nerves,  consequently  the  adult  verte- 
brate body  is  segmented  like  that  of  the  earth-worm.  As  a  worm  grows  by 
the  formation  of  new  segments  at  its  tail-end,  so  the  metameres  of  the  vertebrate 
embryo  begin  to  form  in  the  head  and  are  added  tailwards.  There  is  this  dif- 
ference between  the  segments  of  the  worm  and  the  vertebrate  embryo.  The  seg- 
mentation of  the  worm  is  complete,  while  that  of  the  vertebrate  is  incomplete 
ventrally. 

GROWTH  AND  DIFFERENTIATION  OF  THE  EMBRYO 

A  multicellular  embryo  develops  by  the  division  of  the  fertilized  ovum  to 
form  daughter  cells.  These  are  at  first  similar  in  structure  and,  if  separated,  any 
one  of  them  may  develop  into  a  complete  embryo,  as  has  been  proved  by  the 
experiments  of  Driesch  on  the  ova  of  the  sea-urchin.  The  further  development  of 
the  embryo  depends  (1)  upon  the  multiplication  of  its  cells  by  division;  (2)  upon 
the  growth  in  size  of  the  individual  cells;  (3)  upon  changes  in  their  form  and 
structure. 

The  first  changes  in  the  form  and  arrangement  of  the  cells  give  rise  to  three 


GROWTH    AND   DIFFERENTIATION   OF   THE    EMBRYO  13 

definite  plates,  or  germ-layers,  which  arc  termed  from  their  positions  the  ectoderm 
(outer  skin),  mesoderm  (middle  skin)  and  entoderm  (inner  skin).  In  function 
the  ectoderm,  as  it  covers  the  body,  is  primarily  protective,  and  gives  rise  to  the 
nervous  system  through  which  sensations  are  received  from  the  outer  world.  The 
entoderm,  on  the  other  hand,  lines  the  digestive  canal  and  is  from  the  first  nutritive 
in  function.  The  mesoderm,  lying  between  the  other  two  layers,  naturally  per- 
forms the  functions  of  circulation,  of  muscular  movement  and  of  excretion;  it 
gives  rise  also  to  the  skeletal  structures  which  support  the  body.  While  all  three 
germ-layers  form  definite  sheets  of  cells  known  as  epilhelia,  the  mesoderm  takes 
also  the  form  of  a  diffuse  network  of  cells,  the  mesenchymes. 

The  Anlage. — This  German  word  is  the  term  applied  to  the  first  ag- 
gregation of  cells  which  will  form  any  distinct  part  or  organ  of  the  embryo. 
The  various  anlages  are  differentiated  from  the  germ-layers  by  a  process  of  un- 
equal growth.  At  points  where  multiplication  of  the  cells  is  more  rapid  than  in  the 
circular  area  surrounding  them,  outgrowths  or  ingrowths  of  the  germ-layer  will 
take  place.  The  outgrowths  or  cvaginations  are  illustrated  by  the  development 
of  the  finger-like  villi  from  the  entoderm  of  the  intestine;  ingrowths  or  invagina- 
tions by  the  formation  of  the  glands  at  the  bases  of  the  villi.  According  to  Minot, 
the  development  of  cvaginations  and  invaginations,  due  to  unequal  rapidity  of  growth, 
is  the  essential  factor  in  moulding  the  organs,  and  hence  the  body  of  the  embryo. 

Differentiation  of  Tissues. — The  cells  of  the  germ-layers  which  form 
organic  anlages  may  be  at  first  alike  in  structure.  Thus  the  evagination 
which  forms  the  anlage  of  the  arm  is  composed  of  a  single  layer  of  like 
ectodermal  cells,  surrounding  a  central  mass  of  diffuse  mesenchyma  (Fig. 
131).  Gradually  the  ectodermal  cells  multiply,  change  their  form  and  structure 
and  give  rise  to  the  layers  of  the  epidermis.  By  more  profound  structural  changes 
the  mesenchymal  cells  also  are  transformed  into  the  elements  of  connective  tissue, 
tendon,  cartilage,  bone  and  muscle,  aggregations  of  modified  cells  which  are  known 
as  tissues.  The  development  of  modified  tissue  cells  from  the  undifferentiated 
cells  of  the  germ-layers  is  known  as  histogenesis.  During  histogenesis  the  struc- 
ture and  form  of  each  tissue  cell  are  adapted  to  the  performance  of  some  special 
function  or  functions.  Cells  which  have  once  taken  on  the  structure  and  func- 
tions of  a  given  tissue  can  not  give  rise  to  ceils  of  any  other  type.  In  tissues  like 
the  epidermis,  certain  cells  retain  their  primitive  embryonic  characters  throughout 
life  and,  by  continued  cell-division,  produce  new  layers  of  cells  which  are  later 
comified.  In  other  tissues  all  of  the  cells  are  differentiated  into  the  adult  type 
and,  during  life,  no  new  cells  are  formed.  This  takes  place  in  the  case  of  the 
nervous  elements  of  the  central  nervous  svstem. 


14  INTRODUCTION 

Throughout  life,  tissue  cells  are  undergoing  retrogressive  changes.  In  this 
way  the  cells  of  certain  organs  like  the  thymus  gland  and  mesonephros  degenerate 
and  largely  disappear.  The  cells  of  the  hairs  and  the  surface  layer  of  the  epider- 
mis become  cornified  and  eventually  are  shed.  Tissue  cells  may  thus  normally 
constantly  be  destroyed  and  replaced  by  new  cells. 

The  Law  of  Biogenesis. — Of  great  theoretical  interest  is  the  fact,  con- 
stantly observed  in  studying  embryos,  that  the  individual  in  its  develop- 
ment recapitulates  the  evolution  of  the  race.  This  law  of  recapitulation  was 
asserted  by  Meckel  in  1881  and  was  termed  by  Haeckel  the  law  of  Mo  genesis. 
According  to  this  law,  the  fertilized  ovum  is  compared  to  a  unicellular  organism 
like  the  amoeba;  the  blastula  embryo  is  supposed  to  represent  an  adult  Volvox; 
the  gastrula,  a  simple  sponge;  the  segmented  embryo  a  worm-like  stage,  and  the 
embryo  with  gill-slits  may  be  regarded  as  a  fish-like  stage.  The  blood  of  the 
human  embryo  in  development  passes  through  stages  in  which  its  corpuscles 
resemble  in  structure  those  of  the  fish  and  reptile ;  the  heart  is  at  first  tubular,  like 
that  of  the  fish;  the  kidney  of  the  embryo  is  like  that  of  the  amphibian,  as  are 
also  the  genital  ducts.  Many  other  examples  of  this  law  may  readily  be  observed. 
A  more  complete  account  of  the  general  conceptions  of  embryology  is  given  in 
Minot's  "Laboratory  Textbook  of  Embryology." 

Methods  of  Study. — Human  embryos  not  being  available  for  individual 
laboratory  work,  we  employ  instead  the  embryos  of  the  lower  animals  which 
best  illustrate  certain  points.  Thus  the  ova  of  Ascaris,  a  parasitic  round  worm, 
are  used  to  demonstrate  the  phenomena  of  mitosis;  the  larvae  of  echinoderms, 
or  of  worms,  are  frequently  used  to  demonstrate  the  segmentation  of  the  ovum 
and  the  development  of  the  blastula  and  gastrula  larvae;  the  chick  embryo  af- 
fords convenient  material  for  the  study  of  the  early  vertebrate  embryo,  of  the 
formation  of  the  germ-layers  and  of  the  embryonic  membranes,  while  the  struc- 
ture of  a  mammalian  embryo,  similar  to  that  of  the  human  embryo,  is  best  ob- 
served in  the  embryos  of  the  pig,  which  are  very  readily  obtained.  An  idea  of 
the  anatomy  of  the  embryos  is  obtained  first  by  examining  the  exterior  of  whole 
embryos  and  studying  dissections  and  reconstructions  of  them.  Finally,  each 
embryo  is  studied  in  serial  sections,  the  level  of  each  section  being  determined  by 
(  omparing  it  with  figures  of  the  whole  embryo. 

Along  with  his  study  of  the  embryos  in  the  laboratory,  the  student  should 
do  a  certain  amount  of  supplementary  reading.     Only  the  gist  of  human  organo- 

-is  is  contained  in  the  following  chapters.  A  very  complete  bibliography 
of  the  subject  is  given  in  Keibel  and  Mall's  "Human  Embryology,"  to  which 


GROWTH    AND    DIFFERENTIATION    "I     THE    EMBRYO  [5 

the  student  is  referred.  Below  are  given  the  titles  of  some  of  the  more  impor- 
tant works  on  vertebrate  and  human  embryology,  to  which  the  student  is 
referred  and  in  some  of  which  supplementary  reading  is  required. 

I  1  I  IIS    FOR    REFERENi  E 

Duval,  M.     Atlas  D'Embryologie.     Masson,  Paris. 

His,  W.    Anatomii'  menschlicher  Embryonen.     Vogel,  Leipzig,  1885. 

Keibel,    F.     Normentafel   zur   Entwicklungsgeschichte   der  Wirbelthiere.     Bd. 

I.  Fischer.  Jena,  1897. 
Keibel   and    Elze.     Normentafel   zur   Entwicklungsgeschichte   des   Menschen, 

Jena.  [908. 
Keibel  and  Mall.     Human  Embryology.     Lippincott,  1910-1912. 
Kollmann,  J.     Handatlas  der  Entwicklungsgeschichte  des  Menschen.     Fischer, 

Jena.  1907. 
Lee,  A.  B.     The  Microtomist's  Vade  Mecum.     Blakiston.  Philadelphia. 
Lewis,  F.  T.     Anatomy  of  a  12  mm.  Pig  Embryo.     Amer.  Jour.  Anat.,  vol.  2. 
Minot,  C.  S.     A  Laboratory  Textbook  of  Embryology. 

Thyng,  F.  W.  The  Anatomy  of  a  7.8  mm.  Pig  Embryo.  Anat.  Record,  vol.  5. 
Wilson,  E.  B.    The  Cell  in   Development  and  Inheritance.      Macmillan,    New 

York. 


CHAPTER  I 

THE  GERM  CELLS:  MITOSIS,  MATURATION  AND  FERTILIZATION 

THE  GERM  CELLS 

The  human  organism  with  its  various  tissues  composed  each  of  aggregations 
of  similar  cells  is,  like  that  of  all  other  vertebrates,  developed  from  the  union 
of  two  germ  cells,  the  ovum  and  spermatozoon. 

The  Ovum. — The  female  germ  cell  or  ovum  is  a  typical  animal  cell  pro- 
duced in  the  ovary  [for  structure  of  typical  cell  see  histologic  texts].  It  is  nearly 
spherical  in  form  and  possesses  a  nucleus  with  nucleolus,  chromatin  network, 
chromatin  knots,  and  nuclear  membrane  (Fig.  i).  The  cytoplasm  of  the  ovum 
is  distinctly  granular,  containing  more  or  less  numerous  yolk  granules  and  a 
minute  centrosome.  The  nucleus  is  essential  to  the  life,  growth,  and  reproduction 
of  the  cell.  The  function  of  the  nucleolus  is  unknown;  the  chromatin  probably 
bears  the  hereditary  qualities  of  the  cell.  The  yolk  granules,  containing  a  fatty 
substance  termed  lecithin,  furnish  nutrition  for  the  early  development  of  the 
embryo.  A  relatively  small  amount  of  lecithin  is  found  in  the  ova  of  mammals, 
the  embryo  developing  within,  and  being  nourished  by,  the  uterine  wall  of  the 
mother.  It  is  much  larger  in  amount  in  the  ova  of  fishes,  amphibia,  reptiles, 
birds,  and  the  primitive  mammalia,  the  eggs  of  which  are  laid  and  develop 
outside  of  the  body.  The  so-called  yolk  of  the  hen's  egg  (Fig.  2)  is  the  ovum 
proper  and  its  yellow  color  is  due  to  the  large  amount  of  lecithin  which  it  con- 
tains. The  albumen,  egg-membrane,  and  shell  of  the  hen's  egg  are  secondary 
envelopes  of  the  ovum. 

The  human  ovum  is  of  small  size,  measuring  from  0.22  to  0.25  mm.  in  diam- 
eter (Fig.  1  A) .  The  cytoplasm  is  surrounded  by  a  relatively  thick  radially  striated 
membrane,  the  zona  pellucida.  The  striated  appearance  of  the  zona  pellucida 
is  said  to  be  due  to  fine  canals  which  penetrate  it  and  through  which  nutriment 
is  carried  to  the  ovum  by  smaller  follicle  cells  during  its  growth  within  the  ovary. 
The  origin  and  growth  of  the  ovum  within  the  ovary  are  known  as  oogenesis,  and 
will  be  described  in  Chapter  VIII.  We  may  state  here  that  each  growing  ovum 
is  at  first  surrounded  by  small  nutritive  cells  known  as  follicle  cells.     These  increase 


iS 


THE   GERM   CELLS:     MITOSIS,    MATURATION  AND   FERTILIZATION 


Fig.  i  A. 


Vitelline  '.'/ 
membrane 


Nude, 


Zona. 


2y    pellucida. 


8*"^ 


asm 
'foyiart 


Fig.  i  B. 


Fig.  i. — A,  Human  ovum  examined  fresh  in  the  liquor  folliculi  (Waldeyer) .  The  zona  pellucida  is 
seen  as  a  thick,  clear  girdle  surrounded  by  the  cells  of  the  corona  radiata.  The  egg  itself  shows  a  central 
granular  deutoplasmic  area  and  a  peripheral  clear  layer,  and  encloses  the  nucleus  in  which  is  seen  the 
nucleolus;  B,  ovum  of  monkey.     X  430. 


I  III:    GERM    CELLS 


19 


Fig.  2. — Diagrammatic  longitudinal  section  of  an  un- 
incubated  hen's  egg  (after  Allen  Thomson,  in  Heisler). 
(Somewhat  altered) :  b.l,  germinal  area;  w.y,  white  yolk, 
which  consists  of  a  central  flask-shaped  mass,  and  a  num- 
ber of  concentric  layers  surrounding  the  yellow  yolk 
(y.y.);  v.t,  vitelline  membrane;  x,  a  somewhat  fluid  al- 
buminous layer  which  immediately  envelops  the  yolk; 
IP,  albumen,  composed  of  alternating  layers  of  more  and 
less  fluid  portions;  ch.l,  chalazae;  a.ch,  air-chamber  at 
the  blunt  end  of  the  egg — simply  a  space  between  the  two 
layers  of  the  shell-membrane;  i.s.m,  inner,  s.m,  outer 
layer  of  the  shell-membrane;   s,  shell. 


Fig.  3. — Section  of  human  ovary, 
including  cortex;  a,  germinal  epithel- 
ium of  free  surface;  b,  tunica  albugi- 
nea;  c,  peripheral  stroma  containing 
immature  Graafian  follicles  (d  . 
well-advanced  follicle  from  whose  wall 
membrana  granulosa  has  partially 
separated;  /,  cavity  of  liquor  folli- 
culi;  g,  ovum  surrounded  by  cell-mass 
constituting  cumulus  oophorus  (Pier- 
sol). 


tfi'It^M^a^W" 


■i:  -'mf 


Fig.  4. — Section  of  well-developed  Graafian  follicle 
from  human  embryo  (von  Herff ) ;  the  enclosed  ovum 
contains  two  nuclei. 


Fig.  5. — Ovary  with  mature  Graafian 
follicle  about  ready  to  burst  (Ribcmont- 
Dessaignes). 


20 


THE    GERM    CELLS:      MITOSIS,    MATURATION   AND    FERTILIZATION 


in  number  during  the  growth  of  the  ovum  until  several  layers  surround  it  (Fig.  3). 
A  cavity  appearing  between  these  cells  becomes  filled  with  fluid  and  thus  forms 
a  sac,  the  Graafian  follicle,  within  which  the  ovum  is  eccentrically  located.  The 
cells  of  the  Graafian  follicle  immediately  surrounding  the  ovum  form  the  corona 
radiata  (Fig.  1)  when  the  ovum  is  set  free. 

Ovulation  and  Menstruation. — When  the  ovum  is  ripe,  the  Graafian 
follicle  is  large  and  contains  fluid,  probably  under  pressure.  The  ripe  follicles 
form  bud-like  projections  at  the  surface  of  the  ovary  (Fig.  5),  and  at  these  points 


Ovum 


Follicle  cells 


Zona  pellucida 
Fig.  6. — Immature  follicle  containing  several  ova.     From  the  ovary  of  a  young  monkey.     X  43°- 


the  ovarian  wall  has  become  very  thin.  It  is  probable  that  normally  the  bursting 
of  the  Graafian  follicle  and  the  discharge  of  the  ovum  are  periodic  and  associated 
with  the  phenomena  of  menstruation.  That  ovulation  or  discharge  of  the  ovum 
from  the  ovary  may  occur  independent  of  the  menstrual  periods  has  been  proven 
by  the  observations  of  Leopold.  Also  in  young  girls  ovulation  may  precede 
the  inception  of  menstruation  and  it  may  occur  in  women  some  time  after  the 
menopause. 

At  birth,  or  shortly  after,  all  of  the  ova  are  formed  in  the  ovary  of  the  female 


thk  germ  cells 


2T 


Head 


End 


ring 


Mam  segment 
of  Tail 


>  Galea  capitis 


intend  of  Knob 
Post  end  of  Knob 


it  fiber 


Sheath  of ax/a/ 


thread 


child.     Hensen  estimates  that  a  normal  human  female  may  develop  in  each  ovary 
200  ripe  ova.     Most  of  the  young  ova,  which  may  number  50,000,  degenerate  and 
never  reach  maturity.     At  ovulation  but  one  ovum  is  normally  ripened  and  dis- 
charged   from    the    ovary.      Several 
ova,   however,   may  be  produced  in 
a  single  follicle  in  rare  cases.     Such 
multiple     follicles    have     been     ob- 
served in  human  ovaries  and  are  of 
frequent  occurrence  in  the  ovary  of 
the   monkey.     Fig.  6   shows   such  a 
follicle  containing  live  immature  ova. 

The  Spermatozoon. — The  male 
cell  or  spermatozoon  is  a  minute  cell 
0.05  mm.  long,  specialized  for  active 
movement.  Because  of  their  active 
movements,  spermatozoa  were,  when 
first  discovered,  regarded  as  para- 
sites living  in  the  seminal  fluid.  The 
sperm  cell  is  composed  of  a  flattened 
head,  indistinct  neck  piece,  and  thread- 
like tail  (Fig.  7). 

The  head  is  about  5  micra  in 
length.  It  appears  oval  in  side 
view,  pear-shaped  in  profile.  When 
stained,  the  anterior  two-thirds  of 
the  head  may  be  seen  to  form  a  cap, 
and  the  sharp  border  of  this  cap  is 
the  perforatorium  by  means  of  which 
the  spermatozoon  penetrates  the 
ovum.  The  head  contains  the  nu- 
clear elements  of  the  sperm  cell. 
The  neck  is  said  to  be  disc-shaped 
and  to  contain  the  centrosorhes  as 
the    anterior    and    posterior    centro- 

some  bodies.  The  tail  is  divided  into  a  short  connecting  piece,  aflagellum  which 
forms  about  four-fifths  of  the  length  of  the  sperm  cell  and  a  short  end-piece 
(Fig.  7).     The  connecting  piece  is  marked  off  from  the  flagellum  by  the  annulus. 


-Ax/a/  thread' 
-Capsule 


Terminal  filament 


Fig.  7. — Diagram  of  a  human  spermato- 
zoon, highly  magnified,  in  side  view  (Meves, 
Bonnet). 


22  THE    GERM   CELLS:      MITOSIS,    MATURATION   AND   FERTILIZATION 

It  is  traversed  by  the  axial  filament  (rilum  principale),  and  surrounded  (i)  by 
the  sheath  common  to  the  flagellum;  (2)  by  a  sheath  containing  a  spiral  filament; 
and  (3)  by  a  mitochondria  sheath.  The  flagellum  is  composed  of  an  axial  filament 
surrounded  by  a  cytoplasmic  sheath  and  the  end-piece  is  the  naked  continuation 
of  the  axial  filament. 

The  spermatozoa  are  motile,  being  propelled  by  the  movements  of  the  tail. 
They  swim  always  against  a  current  at  the  rate  of  about  25  micra  per  second,  or 
1  mm.  every  forty  seconds.  This  is  important,  as  the  outwardly  directed  cur- 
rents induced  by  the  ciliary  action  of  the  uterine  tubes  and  uterus  direct  the  sper- 
matozoa by  the  shortest  route  to  the  infundibulum.  Keibel  has  found  sperma- 
tozoa alive  three  days  after  the  execution  of  the  criminal  from  whom  they  were 
obtained.  They  have  been  found  motile  in  the  vagina  twelve  to  seventeen  days 
after  coitus.  They  have  been  kept  alive  eight  days  outside  the  body  by  arti- 
ficial means.  It  is  not  known  for  how  long  a  period  they  may  be  capable  of  fer- 
tilizing ova  but,  according  to  Keibel,  this  period  would  be  certainly  more  than  a 
week. 

MITOSIS  AND  AMITOSIS 

Before  the  discharged  ovum  can  be  fertilized  by  the  male  germ  cell,  it  must 
undergo  a  process  of  cell  division  and  reduction  of  chromosomes  known  as  matu- 
ration. As  the  student  may  not  be  familiar  with  the  processes  of  cell  division, 
a  brief  description  may  be  necessary.  (For  details  of  mitosis  see  text-books  of 
histology  and  E.  B.  Wilson's  "The  Cell".) 

Amitosis. — Cells  may  divide  directly  by  the  simple  fission  of  their  nuclei  and 
cytoplasm.  This  process  is  called  amitosis.  Amitosis  is  said  to  occur  only  in 
moribund  cells.  It  is  the  type  of  cell  division  found  in  the  epithelium  of  the 
bladder. 

Mitosis. — In  the  reproduction  of  normally  active  cells,  complicated  changes 
take  place  in  the  nucleus.  These  changes  give  rise  to  thread-like  structures, 
hence  the  process  is  termed  mitosis  (thread)  in  distinction  to  amitosis  (no  thread). 
Mitosis  is  divided  for  convenience  into  four  phases  (Fig.  8). 

Prophase  (Fig.  8,  I— III) . — 1.  The  centrosome  divides  and  the  two  minute 
bodies  resulting  from  the  division  move  apart,  ultimately  occupying  positions  at 
opposite  poles  of  the  nucleus. 

2.  Astral  rays  appear  in  the  cytoplasm  about  each  centreole.  They  radiate 
from  it  and  the  threads  of  the  central  or  achromatic  spindle  are  formed  between 
the  two  asters,  thus  constituting  the  amphiaster  (Fig.  8,  II). 


MI  I'nSIS    AM)    AMI  IOSIS 


23 


III. 


3.  The  nuclear  membrane  and  nucleolus  disappear,  the  nucleoplasm  and 
cytoplasm  becoming  continuous. 

4.  During  the  above  changes  the  chromatic  network  of  the  resting  nucleus 

resolves  itself  into  a  skein  or  spireme,   the  thread  of  which  soon  breaks  up 

into  distinct,  heavily-staining 

bodies,  the  chromosomes.     A  ._. ,  .. ... 

i.  ,'•■  "•-.^  ii-..-''  "••.. 

definite  number  of  chromo- 
somes is  always  found  in  the 
cells  of  a  given  species.  The 
chromosomes  may  be  block- 
shaped,  rod-shaped,  or  bent 
in  the  form  of  a  U. 

5.  The  chromosomes  ar- 
range themselves  in  the  equa- 
torial plane  of  the  central 
spindle.  If  U-shaped  the 
base  of  each  U  is  directed 
toward  a  common  center. 
The  am  phi  aster  and  the  chro- 
mosomes together  constitute  a 
mitotic  figure  and  at  the  end 
of  the  prophase  this  is  called 
a  monaster. 

Metaphase. — The  longi- 
tudinal splitting  of  the  chro- 
mosomes into  exactly  similar 
halves  constitutes  the  meta- 
phase (Fig.  8.  IV.  V).  The 
aim  of  mitosis  is  thus  accom- 
plished, an  accurate  division 
of  the  chromatin  between  the 
nuclei  of  the  daughter  cells. 

Anaphase. — At  this  stage  the  two  groups  of  daughter  chromosomes  separate 
and  move  up  along  the  central  spindle  fibers,  each  toward  one  of  the  two  asters. 
Hence  this  is  called  the  diastcr  stage  (Fig.  8,  VI).  At  this  stage,  the  centrioles 
may  each  divide  in  preparation  for  the  next  division  of  the  daughter  cells. 

Telophase  (Fig.  8,  VII.  VIII). — 1.  The  daughter  chromosomes  resolve  them- 


K\ 


VII. 


VIII.  .--' 


Fig.  8. — Diagram  of  the  phases  of  mitosis  (Schafer). 


24  THE    GERM   CELLS:      MITOSIS,    MATURATION  AND   FERTILIZATION 

selves  into  a  reticulum  and  daughter  nuclei  are  formed.  2.  The  cytoplasm  di- 
vides in  a  plane  perpendicular  to  the  axis  of  the  mitotic  spindle.  Two  complete 
daughter  cells  have  thus  arisen  from  the  mother  cell. 

The  complicated  processes  of  mitosis,  by  which  cell  division  is  brought  about 
normally,  seem  to  serve  the  purpose  of  accurately  dividing  the  chromatic  sub- 
stance of  the  nucleus  in  such  a  way  that  the  chromatin  of  each  daughter  cell  may 
be  the  same  qualitatively  and  quantitatively. 

This  is  important  if  we  assume  that  the  chromatic  particles  of  the  chromosomes  bear  the 
hereditary  qualities  of  the  cell.  The  number  of  chromosomes  is  constant  in  the  sexual  cells 
of  a  given  species.  The  number  for  the  human  cell  is  in  doubt.  It  has  been  given  as  16,  24, 
and  32.  According  to  Winiwarter's  recent  work,  the  number  of  chromosomes  in  each  immature 
ovum  or  oocyte  is  48,  in  each  spermatogone  47.  Wiemann  (Amer.  Jour.  Anat.,  vol.  14,  p.  461) 
finds  the  number  of  chromosomes  in  various  human  somatic  cells  varies  from  34  to  38.  In 
species  of  A  scaris  megalocephala,  a  parasitic  worm,  but  two  or  four  chromosomes  are  found 
and  in  their  cells  the  processes  of  mitosis  are  most  easily  observed. 

We  have  seen  that  reproduction  in  mammals  is  dependent  upon  the  union  of 
male  and  female  germ  cells.  The  union  of  two  germinal  nuclei  (pronuclei) 
would  necessarily  double  the  number  of  chromosomes  in  the  fertilized  ovum  and 
also  the  number  of  hereditary  qualities  which  their  particles  are  supposed  to  bear. 
This  multiplication  of  hereditary  qualities  is  prevented  by  the  processes  of  matu- 
ration which  take  place  in  both  the  ovum  and  spermatozoon. 


MATURATION 

Maturation  may  be  defined  as  a  process  of  cell-division  during  which  the 
number  of  chromosomes  in  the  germ  cells  is  reduced  to  one-half  the  number 
characteristic  for  the  species. 

The  spermatozoa  take  their  origin  in  the  germinal  epithelium  of  the  testis. 
Their  development,  or  spermatogenesis,  may  be  studied  in  the  testis  of  the  rat; 
their  maturation  stages  in  the  testis  tubes  of  Ascaris.  Two  types  of  cells  may  be 
recognized  in  the  germinal  epithelium  of  the  seminiferous  tubules,  the  sustentacu- 
lar  cells  (of  Sertoli),  and  the  male  germ  cells  or  spermatogonia  (Fig.  9).  The 
spermatogonia  divide,  one  daughter  cell  forming  what  is  known  as  a  primary 
spermatocyte.  The  other  daughter  cell  persists  as  a  spermatogone  and,  by  con- 
tinued division  during  the  sexual  life  of  the  individual,  gives  rise  to  other  primary 
spermatocytes.  The  primary  spermatocytes  correspond  to  the  ova  before  matu- 
ration. Each  contains  the  number  of  chromosomes  typical  for  the  male  of  the 
species.     The  process  of  maturation  consists  in  two  cell  divisions  of  the  primary 


MATURATION 


25 


Fig.  9. — Diagram  showing  cycle  of  phases  in  the  spermatogenesis  of  the  rat  (Schafcr,  Brown). 
The  numbered  segments  of  the  circle  represent  portions  of  different  seminiferous  tubules,  a,  spermato- 
gonia; a',  sustentacular  cells;  b,  spermatocytes  actively  dividing  in  5;  c,  spermatids  forming  an  irregular 
clump  in  1,6,  7  and  8  and  connected  to  sustentacular  cell  a'  in  2,  3,  4  and  5.  In  6,  7  and  8  advanced 
spermatozoa  of  one  generation  are  seen  between  spermatids  of  the  next  generation,  s',  parts  of  sperma- 
tids which  disappear  when  sperms  are  fully  formed;  s,  seminal  granules  representing  disintegration  of  s'; 
a",  in  1  and  2  are  atrophied  sustentacular  cells. 


C  JE  F 

Fig.  10. — Diagrams  of  the  development  of  spermatozoa  (after  Meves  in  Lewis-Stohr) ;  a.c,  an- 
terior centrosome;  a.f.,  axial  filament;  c.p.,  connecting  piece;  ch.p.,  chief  piece;  g.c,  galea  capitis;  n, 
nucleus;  nk.,  neck;  p.,  protoplasm;  p.c,  posterior  centrosome. 


26 


THE    GERM   CELLS:      MITOSIS,    MATURATION   AND   FERTILIZATION 


spermatocytes,  each  producing  first,  two  secondary  spermatocytes,  and  these 
in  turn  four  cells  known  as  spermatids.  During  these  cell  divisions  the  number 
of  chromosomes  is  reduced  to  half  the  original  number,  the  spermatids  possessing 
just  half  as  many  chromosomes  as  the  spermatogonia.     Each  spermatid  now  be- 


Mm®         $%&$m 

mm  r  =  • 

H 


:  c.. 


o 

i. 


A' 


i. 


Fig.  ii. — Reduction  of  chromosomes  in  spermatogenesis  in  Ascaris  megalocephala  (bivalens) 
(Brauer,  Wilson).  A-G,  successive  stages  in  the  division  of  the  primary  spermatocyte.  The  original 
reticulum  undergoes  a  very  early  division  of  the  chromatin  granules  which  then  form  a  doubly  split 
spireme  (B).  This  becomes  shorter  (C)  and  then  breaks  in  two  to  form  two  tetrads  (D)  in  profile,  (E,  in 
end).  F,  G,  H,  first  division  to  form  two  secondary  spermatocytes,  each  receiving  two  dyads.  /, 
secondary  spermatocyte.  /,  K,  the  same  dividing.  L,  two  resulting  spermatids,  each  containing  two 
single  chromosomes. 


comes  transformed  into  a  mature  spermatozoon  (Fig.  10),  the  nucleus  forming  the 
larger  part  of  the  head,  the  centrosome  dividing  and  lying  in  the  neck  or  middle 
piece.  The  posterior  centrosome  is  prolonged  to  form  the  axial  filament,  and  the 
cytoplasm  forms  the  sheaths  of  the  middle  piece  and  tail. 


M  Ml   R Wins 


27 


The  way  in  which  the  number  of  chromosomes  is  redui  ed  may  be  seen  in  the 

spermatogenesis  of  Ascaris  (Fig.  n).  Four  chromosomes  are  typical  fir  Ascaris 
megaloccphala  bivalcns  and  each  resting  primary  spermatocyte  contain-,  this 
number.  When  the  first  maturation  spindle  appears  only  two  chromosomes  are 
formed,  but  each  of  these  is  double,  so  four  are  really  present.  Each  represent- 
the  union  of  two  chromosomes,  shows  a  quadruple  structure,  and  is  termed  a 
tetrad  (Fig.  11  -E,  F).  At  the  metaphase  (G)  the  two  tetrads  split  each  into  two 
chromosomes  which  already  show  evidence  of  longitudinal  fission  and  are  termed 
dyads.     One  pair  of  dyads  goes  to  each  of  the  daughter  cells,  or  secondary  sper- 


Spermatogonium 


Proliferation 
period 


Growth 
period 


Maturation 
period 


12  3  4  1234 

Fig.  12. — Diagrams  of  maturation,  spermatogenesis  and  oogenesis  (Boveri). 


matocytes  (Fig.  n  G,  I).  Before  the  formation  of  a  nuclear  membrane,  the 
second  maturation  spindle  appears  at  once,  the  two  dyads  split  into  four  monads, 
and  each  daughter  spermatid  receives  two  single  chromosomes,  or  one-half  the 
number  characteristic  for  the  species.  A  diagram  of  maturation  in  the  male  As- 
caris is  shown  in  Fig.  12  A.  The  first  maturation  division  is  reductionaL  each 
daughter  nucleus  receiving  two  complete  cJiromosomes  of  the  original  four,  whereas 
in  the  second  maturation  division  as  in  ordinary  mitosis,  each  daughter  nucleus 
receives  a  half  of  each  of  the  two  chromosomes,  these  being  split  lengthwise. 
In  the  latter  case  the  division  is  equational,  each  daughter  nucleus  receiving  chro- 
mosomes bearing  similar  hereditary  qualities.     In  many  insects  and  some  ver- 


28  THE    GERM   CELLS:      MITOSIS,    MATURATION   AND   FERTILIZATION 

tebrates  it  has  been  shown  that  the  number  of  chromosomes  in  the  oogonia  is  even, 
the  number  in  the  spermatogonia  odd,  and  that  all  the  mature  ova  and  half  the 
spermatids  contain  an  extra  or  accessory  chromosome  (see  p.  32). 

Previous  to  fertilization,  the  ova  undergo  a  similar  process  of  maturation. 
Two  cell  divisions  take  place  but  with  this  difference,  that  the  cleavage  is  un- 
equal and,  instead  of  four  cells  of  equal  size  resulting,  there  are  formed  one  large 
ripe  ovum  or  oocyte  and  three  rudimentary  or  abortive  ova  known  as  polar  bodies 
or  polocytes.  The  number  of  chromosomes  is  reduced  in  the  same  manner  as  in 
the  spermatocyte,  so  that  the  ripe  ovum  and  each  polar  body  contain  one-half 
the  number  of  chromosomes  found  in  the  immature  ovum  or  primary  oocyte. 
The  female  germ  cells,  from  which  new  ova  are  produced  by  cell  division,  are 
called  oogonia  and  their  daughter  cells  after  a  period  of  growth  within  the  ovary 
are  the  primary  oocytes,  comparable  to  the  primary  spermatocytes  of  the  male. 
During  maturation  the  ovum  and  first  polocyte  are  termed  secondary  oocytes 
(comparable  to  secondary  spermatocytes) ,  the  mature  ovum  and  second  polocyte, 
with  the  daughter  cells  of  the  first  polocyte,  are  comparable  to  the  spermatids 
(see  diagram  B,  Fig.  12).  Each  spermatid,  however,  may  form  a  mature  sper- 
matozoon, but  only  one  of  the  four  daughter  cells  of  the  primary  oocyte  becomes  a 
mature  ovum.  The  three  polocytes  are  abortive  and  degenerate  eventually, 
though  it  has  been  shown  that  in  the  ova  of  some  insects  the  polar  body  may  be 
fertilized  and  segment  several  times  like  a  normal  ovum.  The  maturation  of 
human  ova  has  not  been  observed,  but  such  a  process  probably  takes  place.  The 
reduction  of  the  chromosomes  may  be  best  observed  in  the  ova  of  Ascaris  and  of 
insects.  The  mouse  offers  a  favorable  opportunity  for  studying  the  maturation 
of  a  mammalian  egg  as  the  ova  are  easily  obtained.  Their  maturation  stages 
have  recently  been  studied  by  Mark  and  Long  (Carnegie  Inst.  Publ.  No.  142). 

Maturation  of  the  Mouse  Ovum. — The  nucleus  of  the  mature  ovum  is  known 
as  the  female  pronucleus.  When  the  spermatozoon  penetrates  the  mature  ovum 
it  loses  its  tail  and  its  head  becomes  the  male  pronucleus.  The  aim  and  end  of 
fertilization  consists  in  the  union  of  the  chromatic  elements  contained  in  the  male 
and  female  pronuclei  and  the  initiation  of  cell  division.  In  the  mouse,  the  first 
polocyte  is  formed  while  the  ovum  is  still  in  the  Graafian  follicle.  In  the  forma- 
tion of  the  maturation  spindle  no  astral  rays  and  no  typical  centrosomes  have 
been  observed.  The  chromosomes  are  V-shaped.  The  first  polar  body  is  seg- 
mented from  the  ovum  and  lies  beneath  the  zona  pellucida  as  a  spherical  mass 
about  25  micra  in  diameter  (Fig.  13).  Both  ovum  and  polar  body  (secondary 
oocytes)  contain  10  or  12  chromosomes,  or  half  the  number  normal  for  the  mouse. 


MATURATION 


29 


(According  to  Mark  and  Long,  the  chromosomes  number  20.)     The  first  matura- 
tion division  is  the  reductional  one  and  the  chromosomes  take  the  form  of  tetrads. 
After  ovulation  has  taken  place,  the  ovum  lies  in  the  ampulla  of  the  uterine 
tube.     If  fertilization  takes  place,  a  second  polocyte  is  cut  off,  the  nucleus  of  the 


/•; 


G 


Fig.  13. — Maturation  and  fertilization  of  the  ovum  of  the  mouse.  A.C-J,  X  500;  B  X  750.  (after 
Sobotta).  A-C,  entrance  of  the  spermatozoon  and  formation  of  the  second  polar  body.  D-E,  develop- 
ment of  the  pronuclei.     F-J,  successive  stages  in  the  first  division  of  the  fertilized  ovum. 


ovum  forming  no  membrane  between  the  production  of  the  first  and  second  polar 
bodies  (Fig.  13  A-D).  The  second  maturation  spindle  and  second  polar  body  are 
smaller  than  the  first.  Immediately  after  the  formation  of  the  second  polar 
body,  the  chromosomes  resolve  themselves  into  a  reticulum  and  the  female  pro- 
nucleus is  formed  (Fig.  13  D). 


30  THE    GERM    CELLS:      MITOSIS,    MATURATION    AND    FERTILIZATION 

Fertilization  of  the  Mouse  Ovum. — Normally,  a  single  spermatozoon  enters 
the  ovum  six  to  ten  hours  after  coitus.  While  the  second  polar  body  is  forming, 
the  spermatozoon  penetrates  the  ovum  and  loses  its  tail.  Its  head  is  converted 
into  the  male  pronucleus  (Fig.  13  D).  The  pronuclei,  male  and  female,  approach 
each  other  and  resolve  themselves  into  a  spireme  stage,  then  into  two  groups  of  12 
chromosomes.  A  centrosome,  possibly  that  of  the  male  cell,  appears  between 
them,  divides  into  two,  and  soon  the  first  segmentation  spindle  is  formed.  The  12 
male  and  12  female  chromosomes  arrange  themselves  in  the  equatorial  plane  of 
the  spindle,  thus  making  the  original  number  of  24  (Fig.  13  I).  Fertilization  is 
now  complete  and  the  ovum  divides  in  the  ordinary  way.  The  fundamental 
results  of  the  process  of  fertilization  are  (1)  the  union  of  the  male  and  female 
chromosomes,  (2)  the  initiation  of  cell  division  or  cleavage  of  the  ovum. 

These  two  factors  are  separate  and  independent  phenomena.  It  has  been  shown  by 
Boveri  and  others  that  fragments  of  sea-urchin's  ova  containing  no  part  of  the  nucleus  may  be 
fertilized  by  spermatozoa,  segment  and  develop  into  larvae.  The  female  chromosomes  are  thus 
not  essential  to  the  process  of  segmentation.  Loeb,  on  the  other  hand,  has  shown  that  the  ova 
of  invertebrates  may  be  made  to  segment  by  chemical  and  mechanical  means  without  the 
cooperation  of  the  spermatozoon.  It  is  well  known  that  the  ova  of  certain  invertebrates  develop 
normally  with  or  without  fertilization  (parthenogenesis).  These  facts  show  that  the  union  of 
the  male  and  female  pronuclei  is  not  the  means  of  initiating  the  development  of  the  ova.  In 
all  vertebrates  it  is,  nevertheless,  the  end  and  aim  of  fertilization. 

Lillie  {Science,  vols.  36  and  38,  pp.  527-530  and  524-528)  has  recently  shown  that  the  cortex 
of  sea-urchin's  ova  produces  a  substance  which  he  terms  fertilizin.  This  substance  he  regards 
as  an  amboceptor  essential  to  fertilization  with  one  side  chain  which  agglutinates  and  attracts 
the  spermatozoa,  another  side  chain  which  activates  the  cytoplasm  and  initiates  the  segmen- 
tation of  the  ovum. 

Spermatozoa  may  enter  the  mammalian  ovum  at  any  point.  If  fertilization 
is  delayed  and  too  long  a  period  elapses  after  ovulation,  the  ovum  may  be  weak- 
ened and  allow  the  entrance  of  several  spermatozoa.  This  is  known  as  poly- 
spermy. 

The  fertilization  of  the  human  ovum  has  not  been  observed,  but  probably  takes  place  in 
the  uterine  tube  some  hours  after  coitus.  Ova  may  be  fertilized  and  start  developing  before  they 
enter  the  uterine  tube.  If  they  attach  themselves  to  the  peritoneum  of  the  abdominal  cavity, 
they  give  rise  to  abdominal  pregnancies.  If  the  ova  develop  within  the  uterine  tube  tubular 
pregnancies  result.  Normally,  the  embryo  begins  its  development  in  the  uterine  tube,  thence 
passes  into  the  uterus  and  becomes  embedded  in  the  uterine  mucosa.  The  time  required  for  the 
passage  of  the  ovum  from  the  uterine  tube  to  the  uterus  is  unknown.  It  probably  varies  in 
different  cases  and  may  occupy  a  week  or  more.  The  ovum  may  in  some  cases  be  fertilized 
within  the  uterus.  Fertilization  is  favored  by  the  fact  that  the  spermatozoa  swim  always 
against  a  current.  As  the  cilia  of  the  uterus  and  uterine  tube  beat  downward  and  outward  the 
sperms  are  directed  upward  and  inward.  They  may  reach  the  uterine  tubes  within  two  hours 
of  a  normal  coitus. 


DETERMINATION    OF    SEX  31 

Usually  but  one  human  ovum  is  produced  and  fertilized  at  coitus.  The  de- 
velopment of  two  or  more  embryos  within  the  uterus  may  be  due  to  the  ripening 
and  expulsion  of  an  equal  number  of  ova  at  ovulation,  these  being  fertilized  later. 
Identical  twins  are  regarded  as  arising  from  the  daughter  cells  of  a  fertilized  ovum, 
these  cells  having  separated,  and  each  having  developed  like  a  normal  ovum. 

The  Significance  of  Mitosis,  Maturation  and  Fertilization.  —  It.  is  assumed  by 
students  of  heredity  that  the  chromatic  particles  of  the  nucleus  bear  the  hereditary  qualities 
of  the  cell.  During  the  course  of  development  these  particles  are  probably  distributed  to  the 
various  cells  in  a  definite  way  by  the  process  of  mitosis.  The  process  of  fertilization  would 
double  the  number  of  hereditary  qualities  and  they  would  be  multiplied  indefinitely  were  it  not 
for  maturation.  At  maturation  not  only  is  the  number  of  chromosomes  halved,  but  it  is 
assumed  also  that  the  number  of  hereditary  qualities  is  reduced  by  half.  In  the  case  of  the 
ovum,  this  takes  place  at  the  expense  of  three  potential  ova,  the  polocytes,  which  degenerate, 
but  is  to  the  advantage  of  the  single  mature  ovum  which  retains  m^re  than  its  share  of  cytoplasm 
and  nutritive  yolk. 

Mendel's  Law  of  Heredity. — Experiments  show  that  all  hereditary  characters  fall  into  two 
opposing  groups,  which  alternate  with  each  other  and  are  termed  allelomorphs.  As  an  example, 
we  may  take  the  hereditary  tendencies  for  black  and  blue  eyes.  It  is  supposed  that  there  are 
paired  chromatic  particles  in  the  germ  cells  which  bear  these  hereditary  tendencies.  Each 
pair  may  be  composed  of  similar  particles,  both  bearing  black-eyed  tendencies  or  both  blue- 
eyed  tendencies,  or  opposing  particles  may  bear  the  one  black,  the  other  blue-eyed  tendencies. 
It  is  assumed  that  at  maturation  these  paired  particles  are  separated,  and  that  one  only  of  each 
pair  is  retained  in  each  germ  cell,  in  order  that  new  and  favorable  combinations  may  be  formed 
at  fertilization.  In  our  example,  either  a  blue-eyed  or  a  black-eyed  tendency  bearing  partide 
would  be  retained.  At  fertilization  the  segregated  tendency-bearing  particles  of  one  sex  may 
enter  into  new  combinations  with  the  allelomorphs  of  the  other  sex,  combinations  which  may 
be  favorable  to  the  offspring.  Three  combinations  may  be  possible.  If  the  color  of  the  eyes 
is  taken  as  the  hereditary  character,  (1)  two  "black"  germ  cells  may  unite;  (2)  two  "blue'* 
germ  cells  may  unite;  (3)  a  "black"  germ  cell  may  unite  with  a  "blue"  germ  cell.  The  result- 
ing individual  will  be  in  (1)  black-eyed;  in  (2)  blue-eyed;  in  (3)  either  black-eyed  or  blue-eyed, 
according  to  whether  one  or  the  other  tendency  predominated.  Were  the  black-eyed  tendency 
in  (3)  predominant  and  the  resulting  individual  black-eyed,  there  would  still  be  blue-eyed 
bearing  chromatin  particles  in  his  or  her  germ  cells.  In  the  next  generation  these  recessive 
blue-eyed  qualities  may  unite  with  similar  qualities  of  another  black-eyed  individual.  The 
offspring  would  be  blue-eyed,  though  both  the  parents  were  black-eyed. 


DETERMINATION  OF  SEX 
The  assumption  that  the  chromosomes  are  the  carriers  of  hereditary  ten- 
dencies is  borne  out  by  the  observations  of  cytologists  on  the  germ  cells  of  insects 
and  some  vertebrates.  It  has  been  shown  that  in  some  forms  the  nucleus  of  the 
spermatogonia  contain  23  chromosomes,  while  those  of  the  oogonia  contain  24. 
When  maturation  and  reduction  of  the  chromosomes  take  place,  half  of  the  sper- 
matids contain  12  chromosomes,  the  other  half  only  eleven,  while  all  the  oocytes 


32  THE    GERM   CELLS:      MITOSIS,    MATURATION   AND   FERTILIZATION 

and  polocytes  contain  12.  There  is  thus  one  extra  chromosome  in  each  mature 
ovum  and  in  each  of  half  the  spermatozoa.  This  chromosome  is  larger  than  the 
others  in  some  insects,  and  is  termed  the  accessory  chromosome.  McClung  was 
the  first  to  assume  that  the  accessory  chromosome  was  a  sex  determinant.  It  has 
since  been  shown  by  Wilson,  Davis,  and  others  that  the  accessory  chromosome 
carries  the  female  sexual  characters.  When  the  spermatozoan  with  12  chromo- 
somes fertilizes  an  ovum,  the  resulting  embryo  is  a  female,  its  somatic  nuclei 
containing  24  chromosomes.  An  ovum  fertilized  by  a  sperm  cell  containing  only 
n  chromosomes  (without  the  accessory  chromosome)  produces  a  male  with  so- 
matic nuclei  containing  only  23  chromosomes.  Winiwarther  (Arch.  d.  Biol.  Bd. 
27)  has  recently  made  similar  observations  on  the  human  germ  cells  but  they  have 
yet  to  be  confirmed  by  other  investigators.  It  is  probable,  however,  that  sex  is 
transmitted  by  the  human  chromosomes  in  much  the  same  way  as  in  insects. 


CHAPTER  II 

SEGMENTATION  OF  THE  FERTILIZED  OVUM  AND 
ORIGIN  OF  THE  GERM  LAYERS 

SEGMENTATION 

The  processes  of  segmentation,  not  having  been  observed  in  human  -ova, 
must  be  studied  in  other  vertebrates.  It  is  probable  that  the  early  development 
of  all  vertebrates  is,  in  its  essentials,  the  same.  It  is  modified,  however,  by  the 
presence  in  the  ovum  of  large  quantities  of  nutritive  yolk.  In  many  vertebrate 
ova  the  yolk  collects  at  one  end,  the  vegetal  pole.  Such  ova  are  said  to  be 
tdolccithal.  Examples  are  the  ova  of  Amphioxus,  the  frog  and  bird.  When 
very  little  yolk  is  present,  the  ovum  is  said  to  be  alccitlial  (no  yolk).  Examples 
are  the  ova  of  the  higher  mammals  and  man.  The  typical  processes  of  cleavage 
may  be  studied  most  easily  in  the  fertilized  ova  of  invertebrates  (Echinoderms, 
Annelids,  and  Mollusks).  Among  Chordates,  the  early  processes  in  develop- 
ment are  primitive  in  a  fish-like  form  Amphioxus.  The  yolk  modifies  the 
development  of  the  amphibian  and  bird's  egg,  while  the  early  structure  of  the 
mammalian  embryo  can  be  explained  only  by  assuming  that  the  ova  of  the 
higher  Mammalia  at  one  time  contained  a  considerable  amount  of  yolk  like  the 
ovum  of  the  bird  and  of  the  lower  mammals. 

Amphioxus. — The  ovum  is  telolecithal,  but  contains  little  yolk  (Fig.  14). 
About  one  hour  after  fertilization  it  divides  vertically  into  two  nearly  equal  daugh- 
ter cells.  The  process  is  known  as  cell  cleavage,  or  segmentation  and  takes  place 
by  mitosis.  Within  the  same  interval  of  time  the  daughter  cells  cleave  in  the 
same  plane,  forming  four  cells.  Fifteen  minutes  later  a  third  segmentation  takes 
place  in  a  horizontal  plane.  As  the  yolk  is  more  abundant  at  the  vegetal  poles 
of  the  four  cells  the  spindle  lies  nearer  the  animal  pole.  Consequently  in  the  eight- 
celled  stage  the  upper  tier  of  four  cells  is  smaller  than  the  lower  four.  By  suc- 
cessive cleavages,  first  in  the  vertical,  then  in  the  horizontal  plane  a  16-  and  32- 
celled  embryo  is  formed.  The  upper  two  tiers  are  now  smaller  and  a  cavity,  the 
blastocoeL  is  enclosed  by  the  cells.  The  embryo  is  called  a  morula  (mulberry). 
In  subsequent  cleavages,  as  development  proceeds,  the  size  of  the  cells  is  di- 
minished while  the  cavity  enlarges  (Fig.  14).  The  embryo  is  now  a  blast ula, 
3  33 


34 


SEGMENTATION   OF   THE   FERTILIZED   OVUM 


nearly  spherical  in  form  and  about  four  hours  old.    The  cleavage  of  the  Amphioxus 
ovum  is  thus  complete  and  somewhat  unequal. 


PB 


Fig.  14. — Segmentation  of  the  egg  of  Amphioxus,  X  220  (after  Hatschek).  1.  The  egg  before  the 
commencement  of  development;  only  one  polar  body,  P.B,  has  been  seen,  but  from  analogy  with  other 
animals  it  is  probable  that  there  are  really  two  present.  2.  The  ovum  in  the  act  of  dividing,  by  a  vertical 
cleft,  into  two  equal  blastomeres.  3.  Stage  with  four  equal  blastomeres.  4.  Stage  with  eight  blastomeres; 
an  upper  tier  of  four  slightly  smaller  ones  and  a  lower  tier  of  four  slightly  larger  ones.  5.  Stage  with 
sixteen  blastomeres  in  two  tiers,  each  of  eight.  6.  Stage  with  thirty-two  blastomeres,  in  four  tiers,  each 
of  eight;  the  embryo  is  represented  bisected  to  show  the  segmentation  cavity  or  blastocoel,  B.  7.  Later 
stage:  the  blastomeres  have  increased  in  number  by  further  division.  8.  Blastula  stage  bisected  to  show 
the  blastocoel,  B. 


The  Ovum  of  the  Frog. — The  ovum  contains  so  much  yolk  that  the  nucleus  and  most 
of  the  cytoplasm  lies  at  the  upper  or  animal  pole.  The  first  cleavage  spindle  lies  in  this  cyto 
plasm.     The  first  two  cleavage  planes  are  vertical  and  the  four  resulting  cells  are  nearly  equ 


SEGMENTATION 


35 


(Fig.  15).  The  spimlks  for  the  third  cleavage  are  located  near  the  animal  pole  and  the  cleavage. 
takes  place  in  a  horizontal  plane.  As  a  result,  the  upper  four  cells  are  much  smaller  than  the 
lower  four.  The  large  yolk-laden  cells  divide  more  slowly  than  the  upper  small  cells.  At  the 
blastula  stage,  the  cavity  is  small,  and  the  cells  of  the  vegetal  pole  an-  each  many  times  larger 
than  those  at  the  animal  pole.    The  cleavage  of  the  frog's  ovum  is  thus  complete  hut  unequal. 


4  5 

Fig.  15. — Segmentation  of  the  frog's  ovum  (Hatschek  in  Marshall).     B,  segmentation  cavity; 

U,  nucleus. 


Ova  of  Reptiles  and  Birds.— The  ova  of  these  vertebrates  contain  a  large 
amount  of  yolk.     There  is  very  little  pure  cytoplasm  except  at  the  animal  pole 


36  SEGMENTATION   OF   THE   FERTILIZED   OVUM 

and  here  the  nucleus  is  located  (Fig.  2).  When  segmentation  begins,  the  first 
cleavage  plane  is  vertical  but  the  yolk,  being  lifeless  matter,  does  not  cleave. 
The  segmentation  is  thus  incomplete  or  meroblastic.  In  the  hen's  ovum  the  cy- 
toplasm is  divided  by  successive  vertical  furrows  into  a  mosaic  of  cells  which,  as  it 
increases  in  size,  forms  a  cap-like  structure  upon  the  surface  of  the  yolk.  These 
cells  are  separated  from  the  yolk  beneath  by  horizontal  cleavage  furrows,  and 
successive  horizontal  cleavages  give  rise  to  several  layers  of  cells.  The  space 
between  cells  and  yolk  mass  may  be  compared  to  the  blastula  cavity  of  Am- 
phioxus  and  the  frog  (Fig.  17).  The  cellular  disc  or  cap  is  termed  the  germinal 
area  or  disc.  The  yolk  mass  which  forms  the  floor  of  the  blastula  cavity  and  the 
greater  part  of  the  ovum  may  be  compared  to  the  large  yolk-laden  cells  at  the 
vegetal  pole  of  the  frog's  blastula.  The  yolk  mass  never  divides,  but  is  gradu- 
ally used  up  in  supplying  nutriment  to  the  embryo  which  is  developed  from  the 
cells  of  the  germinal  area.  Round  the  periphery  of  the  germinal  area  new  cells 
constantly  form  until  they  surround  the  yolk. 

The  Ovum  of  the  Rabbit. — The  ovum  of  all  the  higher  mammals,  like  that 
of  man,  is  microscopic  in  size  and  nearly  alecithal  (no  yolk).  Its  segmentation 
has  been  studied  in  several  mammals  but  we  shall  take  the  rabbit's  ovum  as  an 
example.  The  cleavage  is  complete  and  nearly  equal  (Fig.  16),  a  cluster  of  nearly 
equal  cells  being  formed  within  the  zona  pellucida.  This  corresponds  to  the 
morula  stage  of  Amphioxus.  Next  an  inner  mass  of  cells  is  formed  which  corre- 
sponds to  the  germinal  area,  or  blastoderm,  of  the  chick  embryo  (Fig.  16).  The 
inner  cell  mass  is  overgrown  by  an  outer  layer  which  we  term  the  troph-ectoderm 
because,  in  mammals,  it  supplies  nutriment  to  the  embryo  from  the  uterine  wall. 
Between  the  outer  layer  and  the  inner  cell  mass  fluid  next  appears,  separating 
them  except  at  the  animal  pole.  As  the  fluid  increases  in  amount,  a  hollow  vesicle 
results,  its  wall  composed  of  the  single-layered  troph-ectoderm  except  where  this 
is  in  contact  with  the  inner  cell  mass.  This  stage  is  known  as  the  germinal  or 
blastodermic  vesicle.  It  is  usually  spherical  or  ovoid  in  form,  as  in  the  rabbit,  and 
probably  this  is  the  form  of  the  human  ovum  at  this  stage.  In  the  rabbit  it  is 
of  macroscopic  size  before  it  becomes  embedded.  Among  Ungulates  (hoofed 
animals)  the  vesicle  is  greatly  elongated  and  attains  a  length  of  several  centi- 
meters, as  in  the  pig. 

If  we  compare  the  mammalian  blastodermic  vesicle  with  the  blastula  stages 
of  Amphioxus,  the  frog  and  the  bird,  it  will  be  seen  that  it  is  to  be  homologized 
with  the  bird's  blastula,  not  with  that  of  Amphioxus  (Fig.  17).  In  each  case 
there  is  an  inner  cell  mass  of  the  germinal  area.     The  troph-ectoderm  of  the 


Out,-,  cell  Outer  cells. 


Polar  bint  it 


Outer  cell- 


h  titer  cells. 


Outer  cells. 


Inner  cell 


( luter  cells. 


FlG.  16. — Diagrams  showing  the  segmentation  of  the  mammalian  ovum  and  the  formation  of  the  blasto- 
dermic vesicle  (Allan  Thomson,  after  van  Beneden). 


THE    FORMATION    <>!•'   THE    l<   rODERM    AND   ENTODERM 


37 


mammal  represents  a  precocious  development  of  cells  which,  in  the  bird,  later 

envelop  the  yolk.  The  cavity  of  the  vesicle  is  to  be  compared,  not  with  the 
blastula  cavity  of  Amphioxus  and  the  frog  but  with  the  yolk  mass  plus  the  rudi- 
mentary blastoeoel  of  the  bird's  ovum.  The  mammalian  ovum,  although  almost 
devoid  of  yolk,  thus  develops  much  like  the  yolk-laden  ova  of  reptiles  and  birds.  It- 
segmentation,  however,  is  complete  and  the  early  stages  in  its  development  arc- 
abbreviated. 

A  B 


Blastula  cavity 


D 


Yolk  cavity 


Fig.  17. — Diagrams  showing  the  blastula?:  A,  of  Amphioxus;  B,  of  frog,  and  C  of  chick;  D,  blastodermic 

vesicle   of   mammal. 


In  Primates,  but  one  stage  in  segmentation  has  been  observed.  This,  a  four-celled  ovum 
of  Macacus  nemestrinus  figured  by  Selenka,  shows  the  cells  nearly  equal  and  oval  in  form. 
This  ovum  was  found  in  the  oviduct  of  the  monkey  and  shows  that,  in  Primates  and  probably 
in  man,  segmentation  as  in  other  mammals  takes  place  normally  in  the  oviducts. 


THE  FORMATION  OF  THE  ECTODERM  AND  ENTODERM 
The  blastula  and  early  blastodermic  vesicle  show  no  differentiation  into 
layers.     Such  differentiation  takes  place  later  in  all  vertebrate  embryos  and  the 
three  primary  germ  layers,  ectoderm,  entoderm  and  mesoderm,  are  formed.     From 
these  three  layers  all  of  the  body  tissues  and  organs  are  derived. 

Gastrulation. — In  the  case  of  Amphioxus  and  amphibia  the  entoderm  is 


38 


SEGMENTATION    OF    THE    FERTILIZED    OVUM 


formed  by  a  process  termed  gastrulation.  The  larger  cells  at  the  vegetal 
the  blastula  either  fold  inward  (invaginate)  or  are  overgrown  by  the  more 
dividing  micromeres.  Eventually  the  invaginating  cells 
obliterate  the  blastula  cavity  and  come  into  contact 
with  the  outer  layer  of  cells  (Fig.  18).  The  new  cavity 
formed  is  the  primitive  gut,  or  archenteron.  The  mouth 
of  this  cavity  is  the  blastopore.  The  outer  layer  of  cells 
is  the  ectoderm,  the  inner,  newly  formed  layer  is  the  ento- 
derm. The  entodermal  cells  are  henceforth  concerned  in 
the  nutrition  and  metabolism  of  the  body.  The  embryo 
is  now  termed  a  Gastrula  (little  stomach). 

The  Origin  of  the  Entoderm  in  Reptilia,  Birds  and 
Mammals. — Here  the  entoderm  arises  in  quite  a  differ- 
ent manner.  Instead  of  a  process  of  gastrulation  by  in- 
vagination of  cells  we  have  first  a  process  of  delamination. 
Cells  are  split  off  or  delaminated  from  the  under  side  of 
the  germinal  area,  arrange  themselves  in  a  definite  inner 
layer,  and  thus  the  yolk  entoderm  is  formed.  This  layer 
is  already  apparent  in  a  longitudinal  section  through  the 
germinal  area  of  a  chick  (Tig.  19).     In  mammals  like  the 


pole  of 
rapidly 


y<* 


-ulation  of  amphioxus  (modified  from  Hatschek). 

.1,  Blastula;  az,  animal  rolls;  vz,  vegetative  cells;  fh,  cleavage-cavity. 

ginning  invagination  of  vegetative  pole.    C,  Ciastrula  stage,  the 

-nation  of  the  v   etative  cells  being  complete;  ak,  outer  germ- 

ik,  inner  germ-layer;   nd,  archenteron;   u,  blastopore  (Heisler). 


"3  '-:      ** 


«V: 


M 

0 

fa 


CHE    FORMATION'   ()F   THE    ECTODERM    AND    ENTODERM 


39 


rabbit,  the  entoderm  is  split  from  the  under  side  of  the  germinal  area  ami 
the  cells  soon  grow  around  the  inside  of  the  blastodermic  vesicle,  and  form 
an  inner  entodermal  sae  (Fig.  16).     In  Tarsius,  a  creature  classed  by  Hubre.  ht 


Ectoderm  of  embryonic  disc 


Blastopore 


Ectoderm 


Yolk  entoderm 


Blastopore 


Ectoderm 


'Completion  plate  " 


Protentoderm 


Yolk  entoderm 
Blastopore 


■J   ,  <  o  •  •    *     i>     t     *,. 
i  •  '    -     — 


Peristomal  mesoderm 


.ff,'.,*.'.  •  •-•-!£££  ■  .'  *fc  ]  / 


"  Completion 
plate  " 


Peristomal  mesoderm 


Completion  plate" 


Peristomal 
mesoderm 


"Completion  plate  " 

Fig.  20. — From  medial  vertical  sections  through  embryonic  disk  of  lizard,  showing  five  successive 

in  gastrulation  (Wenckeback,  Bonnet;. 


with  the  Primates,  the  entoderm  cells,  after  splitting  off,  do  not  grow  around 
the  wall  of  the  vesicle  as  in  the  rabbit,  but  soon  form  an  entodermal  sac 
separated  by  a  space  from  the  troph-ectoderm  layer.     Just  how  the  vesicle  is 


40 


SEGMENTATION    OF    THE    FERTILIZED    OVUM 


Fig.  21. — Two  germ-discs  of  hen's  egg  in  the 
first  hours  of  incubation  (after  Roller  in  Heislerj : 
df,  area  opaca;  /;/,  area  pellucida;  s,  crescent;  sk, 
crescent-knob;  es,  embryonic  shield;  pr,  primitive 
groove. 


formed  is  not  known.  It  is  attached  only  to  the  cells  of  the  germinal  area. 
Although  this  stage  has  not  been  observed  in  the  human  embryo  it  is  probable 
from  the  structure  of  the  youngest  known  human  embryo  that  an  entodermal 

vesicle  is  formed  in  much  the  same 
way  as  in  Tarsius. 

Gastrulation  in  Reptiles  and 
Birds. — After  the  formation  of  the 
primary  or  yolk  entoderm  in  reptiles 
and  birds,  a  process  of  invagination 
takes  place  which  has  been  compared 
to  gastrulation  in  Amphioxus  and 
Amphibia.  In  the  lizard  after  the 
primary  entoderm  has  developed  by 
delamination  a  curved  depression  is 
formed  at  the  posterior  border  of  the  germinal  area.  There  is  a  true  invagina- 
tion of  cells  at  this  point  (Fig.  20).  The  cells  grow  cephalad  and  contain  an 
invagination  cavity.  Later  the  floor  of  the  cavity  fuses  with  the  yolk  entoderm 
and  disappears.  The  cells  of  the  roof  persist  as  the  dorsal  or  notochordal  plate. 
The  crescentic  depression,  at  which  point 
invagination  occurs,  may  be  compared  a 

to  the  blastopore  of  Amphioxus.  The 
invagination  cavity  is  the  gastrula  cavity 
or  archenteron  and  the  dorsal  plate 
represents  the  entodermal  roof  of  the 
gastrula  cavity. 

In  the  bird,  invagination  takes  place 
at  a  crescentic  groove  (Fig.  21),  but  the 
ingrowing  cells  form  a  solid  plate,  at 
first  without  an  invagination  cavity. 
The   result  is  the  same,   however,    the 

formation  of  a  notochordal  plate  which  lies  beneath  the  ectoderm.    The  crescentic 
groove  is  interpreted  as  representing  the  blastopore  (Fig.  19). 

Formation  of  the  Primitive  Streak  in  Birds. — The  germinal  area  increases 
in  siz<  by  growth  about  its  periphery,  new  cells  constantly  being  formed  here  until 
eventually  the  germinal  area  surrounds  the  yolk.  According  to  the  interpreta- 
tion of  Duval  and  Hertwig,  as  the  periphery  of  the  germinal  area  extends  itself, 
a  middle  point  in  the  cranial  lip  of  the  crescentic  groove  remains  fixed  while  the 


"Ns 


Jl 


f/Y 


if      \\\\ 

Fig.  22. — Diagram  elucidating  the  forma- 
tion of  the  primitive  groove  (after  Duval). 
The  increasing  size  of  the  germ-disc  in  the 
course  of  the  development  is  indicated  by  dot- 
ted circular  lines.  The  heavy  lines  represent 
the  crescentic  groove  and  the  primitive  groove 
which  arises  from  it  by  the  fusion  of  the  edges 
of  the  crescent  (Heisler). 


TIIK    FORMATION    OF    THE    ECTODERM    AND    ENTODERM 


41 


edges  of  the  lip  on  each  side  arc  carried  caudad  and  brought  together. 
Thus  the  crescent  is  transformed  into  a  longitudinal  slit,  as  in  Fig.  26. 
The  lips  of  the  slit  fuse,  and  the  line  of  fusion  is  marked  by  the  longi- 
tudinal primitive  groove.  This 
interpretation  of  the  primitive 
groove  and  streak,  shown  in 
Fig.  22,  is  known  as  the  con- 
crescence theory.  According  to 
the  theory,  thecrescentic  groove 
of  reptiles  and  birds  is  homo- 
logous with  the  blastopore  of 
Amphioxus.  As  it  is  trans- 
formed into  the  primitive  streak 
and  groove,  these  represent  a 
modified  blastopore.  Accord- 
ing to  this  view,  a  large  part 
of  the  entoderm  of  birds,  rep- 
tiles, and  mammals  is  formed 
by  gastrulation,  as  in  Amphi- 
oxus. 

Gastrulation  in  Mammals. — As  in  reptiles  and  birds  so  also  in  mammals, 
a  process  resembling  gastrulation  takes  place  but  after  the  formation  of  the  xolk 
entoderm.     A  primitive  streak  appears  at  the  posterior  border  of  the  germinal 


Fig.  23. — The  primitive  streak  of  pis,'  embryos  (Kei- 
bel).  A,  embryo  with  primitive  streak  and  primitive 
node;  B,  a  later  embryo  in  which  the  medullary  groove  is 
also  present,  cephalad  in  position. 


Post  opening  of  notochord  canal 
Pri.-nitivf  streak 


/Intopenina  of        /Int. persisting  portion  of 
notochord  canal       notochordal  canaJ 


Fig.  24. — Median  longitudinal  section  through  the  blastoderm  of  a  bat  (Vcspcrlilio  minimis) 

(after  Van  Beneden). 


area  (Fig.  23),  with  a  crescentic  opacity  corresponding  to  the  crescentic  groove 
of  birds.  Longitudinal  sections  (Fig.  24)  of  the  germinal  area  of  the  bat  show  the 
formation  of  a  dorsal  or  notochordal  plate  which  has  replaced,  and  is  fused  later- 
ally with,  the  yolk  entoderm.     A  blastopore  or  notochordal  canal  is  present  lead- 


42 


SEGMENTATION   OF   THE   FERTILIZED    OVUM 


ing  from  the  dorsal  surface  of  the  germinal  area  into  the  space  beneath  the  ento- 
derm, the  archenteron.  No  gastrulation  stage  for  the  human  embryo  has  yet 
been  observed  but  the  primitive  streak  may  be  recognized  in  later  stages  (Fig. 
73  A).  There  is  also  evidence  of  an  opening,  the  notochordal  or  neat -enteric  canal, 
leading  from  the  exterior  into  the  cavity  of  the  primitive  gut  (archenteron). 

According  to  the  view  of  Keibel  and  Hubrecht,  the  invagination  of  cells  to  form  the  noto- 
chordal plate  in  reptiles,  birds  and  mammals  is  a  secondary  process  not  to  be  compared  with 
formation  of  the  entoderm  by  gastrulation,  as  in  Amphioxus.  The  notochordal  plate  is  not 
entodermal  but  ectodermal,  and  the  primitive  streak  cannot  be  compared  in  its  entirety  to  the 
blastopore  of  Amphioxus. 


THE    ORIGIN    OF    THE    MIDDLE    GERM    LAYER    (Mesoderm),   NOTOCHORD,  AND 

NEURAL  TUBE 

Amphioxus. — The  dorsal  plate  of  entoderm,  which  forms  the  roof  of  the 
archenteron,  gives  rise  to  paired  lateral  diverticula  or  ccelomic  pouches  (Fig.  25). 
These  separate  both  from  the  plate  of  cells  in  the  mid-dorsal  line  (which  form  the 
notochord),  and  from  the  entoderm  of  the  gut,  and  become  the  primary  mesoderm. 


Fig.  25.  Origin  of  the  mesoderm  in  Amphioxus  (after  Hatschek).  n.g.,  neural  groove;  n.c,  neural 
<anal;  e/i.,  anlage  of  aotochord;  mes.  som.,  mesodermal  segment;  eel.,  ectoderm;  cut.,  entoderm;  (7/., 
cavity  of  gut;  coe.,  ccelom  or  body  cavity. 

The  mesodermal  pouches  grow  ventrad  and  their  cavities  form  the  ccelom  or 
body  cavity.  Their  outer  walls,  with  the  ectoderm,  form  the  body  wall  or 
SOtnatopleure;  their  inner  walls  with  the  gut  entoderm,  form  the  intestinal  wall 
(splanchnopleure).  In  the  meantime,  a  dorsal  plate  of  cells  cut  off  from  the  ec- 
todenrj  has  formed  the  neural  tube  (anlage  of  central  nervous  system),  and  the 
DOtochordal  plate  has  become  a  cord  or  cylinder  of  cells  extending  the  length  of 
the  embryo  (axial  skeleton).     In  this  simple  fashion  the  ground  plan  of  the 


THE    ORIGIN    OF    THE    MIDDLE   GERM    LAYER 


43 


Neural 
groove 

A 

Primitive 
node 


Primitive 
groove 

AreaopacaX 


Blood  island 


Fig.  26. — Dorsal  surface  view  of  a  twenty-hour  chick  em- 
bryo showing  primitive  streak  and  extent  of  mesoderm  (after 
Duval).  The  lines  A,  B,  and  C  indicate  the  levels  of  the  cor- 
responding sections  shown  in  Fig.  28. 


chordate  body  is  developed. 
In  Amphibia  from  the  dor- 
sal plate  of  entoderm  the 
mesodermal  diverticula 
grow  out  as  solid  plates 
between  ectoderm  and  en- 
toderm. Later,  these  plates 
split  into  two  layers  and 
the  cavity  so  formed  gives 
rise  to  the  ccelom. 

Origin  of  the  Meso- 
derm in  Chick  Embryos. — 
If  we  examine  a  chick  em- 
bryo of  twenty  hours'  in- 
cubation (Fig.  26),  it  will 
be  seen  that  the  primitive 
streak  is  formed  as  a  linear 
opacity  near  the  posterior 

border  of  the  germinal  area.     Over  a  somewhat  pear-shaped  clear  area  the  yolk 
has  been  dissolved  away  from  the   overlying  entoderm.     This  area,  from  its 

appearance,  is  termed  the  area  pcllucida. 
It  is  surrounded  by  the  darker  and  more 
granular  area  opaca.  Whether  or  not  the 
primitive  streak  represents  the  fused  lips 
of  the  blastopore,  it  is  certain  that  it 
represents  the  point  of  origin  for  the 
middle  germ  layer.  It  also  indicates 
the  future  longitudinal  axis  of  the  em- 
bryo. Proliferation  of  cells  takes  place 
here  between  ectoderm  and  entoderm 
and  there  grows  out  laterally  and  caud- 
ally  between  these  layers  a  solid  plate 
of  mesoderm,  as  in  amphibia.  The 
shaded  area  in  Fig.  26  shows  the  extent 
Fig.  27.— Surface  view  of  a  twenty-one-       of  the  mesoderm.     It  extends   at  first 

hour  chick  embryo,  in  which  the  head-fold  and         mQre  -j,       Q^&^    tQ    ^    primit£ve 

first  pair  of  primitive  mesodermal   segments 

are  present  (after  Duval).  streak,    at    the    cranial    end    of    which 


Blood  island. 


44 


SEGMENTATION   OF    THE   FERTILIZED    OVUM 


appears  a  shaded  thickening,  the  primitive  knot  or  node  (Hensen's).  From 
this  point  it  grows  crania lly,  forming  along  the  midline  a  thicker  layer  of 
tissue,  the  notochordal  plate  or  head  process  (Fig.  26).  At  twenty-five  hours 
(Fig.  31).  the  mesoderm  forms  lateral  wings  which  extend  cephalad  beyond 
the  limits  of  the  area  pellucida.  The  space  between  these  wings  is  the 
proamniotic  area.  A  transverse  section  through  the  primitive  streak  at 
twenty  hours  (see  guide  line  C,  Fig.  26)  shows  the  three  germ  layers  distinct 


Ectod 


Neural  plate 


Mesoderm    /  ^A/otochorda-t  plate  Entoderm 


Ectod > 


Mesoderm 


Primitive  node 


Entoderm 


Ectod 


Primitive  sTreaK 


Mesoderm 

Fig.  28. — Transverse  sections  through  the  embryonic  area  of  a  twenty-hour  chick.     A,  through  the  head 
process;    B,  through  the  primitive  node;    C,  through  the  primitive  streak.     X  165. 


laterally  ('Fig.  28  C).  In  the  midline,  a  depression  in  the  ectoderm  is  the 
primitive  groove.  In  this  region  there  is  no  line  of  demarcation  between 
ectoderm  and  mesoderm.  A  transverse  section  through  the  primitive  node  (Fig. 
28  B.  guide  line  H.  Fig.  26)  shows  in  this  region  the  marked  proliferation  of  cells, 
whir  b  an-  growing  cephalad  to  form  the  notochordal  plate  (head  process). 

A  transverse  section  through  the  notochordal  plate  just  beginning  to  form 
at  this  stage  ('Fig.  28  A,  guide  line  A,  Fig.  26)  shows  the  thickening  near  the 
midline  which  will  separate  from  the  lateral  mesoderm  and  form  the  notochord. 


THE    ORIGIN   OF   THE    MIDDLE    GERM    LAYER  45 

After  the  notochordal  plate  becomes  prominent  at  twenty  hours  the  dif- 
ferentiation of  the  germinal  area  is  rapid.  A  curved  fold,  involving  the  three 
layers  of  the  germinal  area,  is  formed  cephalad  to  the  notochordal  process.  This 
is  the  head-fold  and  is  the  anlage  of  the  head  of  the  embryo  (Fig.  27).  The  ecto- 
derm has  thickened  on  each  side  of  the  mid-dorsal  line,  forming  the  neural  folds. 
The  groove  between  these  is  the  neural  groove.  The  closure  of  this  groove  will 
form  the  neural  tube,  the  anlage  of  the  central  nervous  system.  The  notochord  is 
now  differentiated  from  the  mesoderm  and  may  be  seen  in  the  mid-dorsal  line 
through  the  ectoderm.  In  the  mesoderm  lateral  to  the  notochord  and  cephalad 
to  the  primitive  node,  transverse  furrows  have  differentiated  a  pair  of  mesodermal 
segments.  As  development  proceeds  these  increase  in  number,  successive  pairs 
being  developed  caudally.     They  will  be  described  in  detail  later. 

To  sum  up,  in  the  chick  the  mesoderm  appears  with  the  formation  of  the 
primitive  streak.  It  originates  from  the  primitive  streak  and  node  and  spreads 
in  all  directions  between  the  other  germ  layers  as  an  undivided  plate  of  cells. 
It  grows  cephalad  in  the  midline  as  the  notochordal  process  or  plate  from  which 
the  notochord  is  developed. 

As  the  mesoderm  is  derived  from  the  entoderm  in  Amphioxus,  its  origin  is  generally 
regarded  as  entodermal  in  birds  and  mammals.  This  would  certainly  be  the  case  if  we  interpret 
the  notochordal  process  and  entoderm  as  formed  by  a  process  of  gastrulation.  Keibel  (in 
Kernel  and  Mall,  vol.  I),  however,  holds  that  the  mesoderm  and  notochordal  plate  are  derived 
from  the  ectoderm,  and  that  any  relation  which  they  bear  to  the  entoderm  is  of  secondary  origin. 

The  Origin  of  the  Mesoderm  in  Mammals. — As  we  have  seen,  the  primitive 
streak  is  formed  on  the  surface  of  the  germinal  area  in  mammalian  embryos  as 
in  the  chick.  It  has  been  described  as  due  to  a  keel-like  thickening  of  the  ecto- 
derm, and  the  knob-like  mass  of  cells  at  its  cephalic  end,  the  primitive  node,  is 
the  first  to  appear.  The  mesoderm  is  formed  precisely  as  in  the  chick,  growing 
out  in  all  directions  from  the  primitive  streak  and  node  between  the  other  two 
layers.  Its  extent  in  rabbit  embryos  is  shown  in  Fig.  29  A  and  B.  Cranial 
to  the  primitive  node  the  notochord  is  differentiated  in  the  midline,  the  meso- 
derm being  divided  into  two  wings.  The  mesoderm  rapidly  grows  round  the 
wall  of  the  blastodermic  vesicle  until  it  finally  surrounds  it  and  the  two  wings  fuse 
ventrally  (Fig.  30  A  and  B).  The  single  sheet  of  mesoderm  soon  splits  into  two, 
the  cavity  between  being  the  co:lom  or  body  cavity.  The  outer  mesodermal  layer 
(somatic),  with  the  ectoderm,  forms  the  somatopleure  or  body  wall,  the  inner 
splanchnic  layer,  with  the  entoderm,  forms  the  intestinal  wall  or  splanclinopleure. 
The  neural  tube  having  in  the  meantime  been  formed  from  the  neural  folds  of  the 


46 


SEGMENTATION    OF    THE    FERTILIZED    OVUM 


ectoderm,  we  have  the  ground  plan  of  the  vertebrate  body,  the  same  in  man  as  in 
Amphioxus. 

The  origin  of  the  mesoderm  in  the  human  embryo  is  unknown,  but  in  Tarsius 
it  has  two  sources,     (i)  The  primary  mesoderm  derived  by  delamination  from  the 


Fig.  20. — Diagrams  showing  the  extent  of  the  mesoderm  in  rabbit  embryos  (Kolliker).  In  .4  the 
mesoderm  is  represented  by  the  pear-shaped  area  at  the  caudal  end  of  the  embryonic  area;  in  B  by  the 
circular  area  which  surrounds  the  embryonic  area. 

ectoderm  at  the  caudal  edge  of  the  germinal  area.  This  forms  the  extra-embryonic 
mesoderm  and  takes  no  part  in  forming  the  body  of  the  embryo.  (2)  The 
secondary  or  intraembryonic  mesoderm,  which  gives  rise  to  body  tissues,  takes 


Ectoderm 

Mesoderm 
Entoderm 


Mesodermal  segment 
Ectoderm 


Archenteron 


Entoderm 


Archenteron 


Neural  iube 
/Vephrotome 
Notochord 

Splanchnic 
mesoderm 


Coelom 


I  i'..   jo.— Diagrams  showing  the  origin  of  the  germ  layers  of  mammals  as  seen  in  transverse  section 

(modified  from  Bryce). 


its  origin  from  the  primitive  streak  and  node  as  in  the  chick  and  lower  mammals. 
The  origin  of  the  mesoderm  in  human  embryos  is  probably  much  the  same  as  in 
Tarsius. 

The  Notochord. — In  mammals  and  in  man  the  notochordal  plate  is  described 


THE    ORIGIN    OF    THE    MIDDLE    GERM    LAYER  47 

as  taking  its  origin  directly  from  the  entoderm.  Keibel  points  out  that  this  con- 
nection of  the  notochord  is  only  secondary.  The  notochordal  process  grows 
cephalad  from  the  primitive  node  and  the  tissue  from  which  it  is  derived  is  of 
ectodermal  origin,  according  to  Keibel's  view.  In  later  stages,  the  notochord 
extends  in  the  midline  beneath  the  neural  tube  from  the  tail  to  a  dorsal  out-pocket- 
ing of  the  oval  entoderm  known  as  Seessel's  pocket.  It  becomes  enclosed  in  the 
centra  of  the  vertebrae  and  in  the  base  of  the  cranium,  and  eventually  degenerates. 
In  Amphioxus,  it  forms  the  only  axial  skeleton  and  it  is  persistent  in  the  axial 
skeleton  of  fishes  and  Amphibia.  In  man,  traces  of  it  are  found  as  pulpy  masses 
in  the  intervertebral  discs. 


CHAPTER  III 

THE  STUDY  OF  CHICK  EMBRYOS 

In  the  following  descriptions  we  shall  use  the  terms  dorsad  and  ventrad  to 
indicate  "towards  the  back"  or  "towards  the  belly";  cephalad  and  cranially  to 
denote  "headwards,"  caudad  to  denote  "tailwards,"  and  laterad  when  the  loca- 
tion is  at  the  side.  As  there  is  no  single  word  in  English  to  express  the  primi- 
tive cellular  germ  of  a  structure,  the  German  word  anlage  has  been  adopted  by 
embryologists  and  will  be  used  here. 

Chick  embryos  may  be  studied  whole  and  most  of  the  structures  identified  up  to  the  end 
of  the  second  day.  The  eggs  should  be  opened  in  normal  saline  solution  at  400  C.  With 
scissors,  cut  around  the  germinal  area,  float  the  embryo  off  the  yolk  and  remove  the  vitelline 
membrane.  Then  float  the  embryo  dorsal  side  up  on  a  glass  slide,  remove  enough  of  the  saline 
solution  to  straighten  wrinkles,  and  carefully  place  over  the  embryo  a  circle  of  tissue  paper 
with  opening  large  enough  to  leave  the  germinal  area  exposed.  Add  a  few  drops  of  fixative 
(5  per  cent,  nitric  acid  gives  good  fixation)  and  float  embryo  into  a  covered  dish.  After  fix- 
ing and  hardening,  stain  in  acid  Hematoxylin  (Conklin)  or  in  acid  Carmine.  Extract  sur- 
plus stain,  clear,  and  mount  on  slide  supporting  cover-slip  to  prevent  crushing  the  embryo. 
Acid  Haematoxylin  gives  the  best  results  for  embryos  of  the  first  two  days.  For  a  detailed  ac- 
count of  embryological  technique  see  Lee's  "Microtomist's  Vade  Mecum." 


EMBRYO  OF  SEVEN  SEGMENTS  (TwTiNTY-FIVE  HOURS'  INCUBATION) 
In  this  embryo  (Fig.  31)  there  is  a  prominent  network  of  blood-vessels  and 
blood-cells  in  the  caudal  portion  of  the  area  opaca.  In  its  cranial  portion  isolated 
groups  of  blood  and  blood-vessel  forming  cells  are  seen  as  blood-islands.  To- 
gether, they  constitute  the  angioblast  from  which  arises  the  blood  vascular  system. 
The  area  pellucida  has  the  form  of  the  sole  of  a  shoe  with  broad  toe  directed  for- 
ward. The  head-fold  has  become  cylindrical  and  the  head  of  the  embryo  is  free 
for  a  short  distance  from  the  germinal  area.  The  mesoderm  extends  on  each  side 
beyond  the  head  leaving  a  median  clear  space,  the  proamniotic  area.  The  en- 
toderm is  carried  forward  in  the  head-fold  as  the  fore-gut,  from  which  later  arise 
the  pharynx,  esophagus,  stomach  and  a  portion  of  the  small  intestine.  The 
opening  into  the  fore-gut  faces  caudad  and  is  the  fovea  cardiaca.  The  way  in 
which  the  entoderm  is  folded  up  from  the  germinal  disc  and  forward  into  the 
head  is  seen  well  in  a  longitudinal  section  of  an  older  embryo  (Fig.  32).     The 

48 


EMBRYO   OF   SEVEN    SEGMENTS  49 

tubular  heart  lies  ventral  to  the  fore-gut  and  cranial  to  the  fovea  cardiaca.  In 
later  stages  it  is  bent  to  the  right.  Converging  forward  to  the  heart  on  each  side 
of  the  fovea  are  the  vitelline  veins  just  making  their  appearance  at  this  stage. 

The  lips  of  the  neural  folds  have  met  throughout  the  cranial  two-thirds  of  the 
embryo  but  have  not  fused.  The  neural  tube  formed  thus  by  the  closing  of  the 
ectodermal  folds  is  open  at  either  end.     Cephalad,  the  neural  tube  has  begun  to 


interior  neuropore  Forebram 

■        ■:   m 


Pharynx 


Left  vitelline  M    *t 
vein 


Me  so  derma 
Segment  3 


Irea    \ 
pellucida 


Nolochord 


Area  opaca 
Primitive  sfreaK 


Free  portion  of  head 


^  Fovea  cardiaca 


Bight  vitelline 
vein 


\~y    yf — &-  Neural  groove 


Segmental 


■L  Primitive  node 


island 


Fig.  31. — Dorsal  view  of  a  twenty-hve-hour  chick  embryo  with  seven  primitive  segments.     X  20. 


expand  to  form  the  brain  vesicles.  Of  these  only  the  fore-brain  is  prominent, 
and  from  it  laterally  the  optic  vesicles  are  budding  out.  The  paraxial  mesoderm 
is  divided  by  transverse  furrows  into  seven  pairs  of  primitive  segments.  Caudally 
between  the  segments  and  the  primitive  streak  there  is  undifferentiated  meso- 
derm, but  new  pairs  of  segments  will  develop  in  this  region.  Looking  through  the 
open  neural  tube  (rhomboidal  sinus),  one  may  see  in  the  midline  the  chorda  dor- 
salis  extending  from  the  primitive  node  cephalad  until  it  is  lost  beneath  the 
4 


5° 


THE    STUDY    OF    CHICK   EMBRYOS 


neural  tube  in  the  region  of  the  primitive  segments.  The  primitive  streak  is 
still  prominent  at  the  posterior  end  of  the  area  pellucida,  forming  about  one-fourth 
the  length  of  the  embryo.     Transverse  sections  through  the  primitive  streak 

and  open  neural  groove  show  approximately  the 
same  conditions  as  in  the  twenty-hour  embryo 
(Figs.  26  and  28). 

A  Transverse  Section  through  the  Fifth 
Primitive  Segment  (Fig.  33)  is  characterized  by 
the  differentiation  of  the  mesoderm,  the  approxi- 
mation of  the  neural  folds  and  the  presence  of  two 
vessels,  the  descending  aortae  on  each  side  between 
the  mesodermal  segments  and  the  entoderm.  The 
neural  folds  are  thick  and  the  ectoderm  is  thickened 
over  the  embryo.  The  notochord  is  a  sharply  de- 
fined oval  mass  of  cells.  The  mesodermal  segments 
are  somewhat  triangular  in  outline  and  connected 
by  the  intermediate  cell  mass  with  the  lateral  meso- 
derm. This  is  partially  divided  by  irregular  flat- 
tened spaces  into  two  layers,  the  dorsal  of  which  is 
the  somatic,  the  ventral  the  splanchnic  layer  of 
mesoderm.  Later,  the  spaces  unite  on  either  side 
to  form  the  ccelom  or  primitive  body  cavity. 

Transverse  Section  Caudal  to  the  Fovea 
Cardiaca  (Fig.  34). — The  section  is  characterized 
(1)  by  the  closing  together  of  the  neural  folds  to 
form  the  neural  tube;  (2)  by  the  dorsad  and  laterad 
folding  of  the  entoderm  which,  a  few  sections  nearer 
the  head  end,  forms  the  fore-gut  or  pharynx;  (3) 
by  the  presence  of  the  vitelline  veins  laterally  be- 
tween the  entoderm  and  mesothelium;  (4)  by  the 
wide  separation  of  the  somatic  and  splanchnic 
mesoderm  and  the  consequent  increase  in  the 
size  of  the  ccelom.  In  this  region,  it  later  sur- 
rounds the  heart  and  forms  the  pleuro- pericardial 
cavity. 
The  neural  tube  in  this  region  forms  the  third  brain  vesicle  or  hind-brain. 
The  neural  folds  have  not  yet  fused.     Mesodermal  segments  do  not  develop  in 


Fig.  32. — Median  longitu- 
dinal section  of  a  thirty-six-hour 
chick  embryo  (Marshall).  AN, 
amnion  fold;  BF,  fore-brain;  BH, 
hind-brain;  BM,  mid-brain;  CII, 
notochord;  CP,  pericardial  cav- 
ity; GF,  fore-gut;  //,  entoderm; 
.V.S',  spinal  cord;  NT,  neurenteric 
canal;  PS,  primitive  streak;  RV, 
ventricle  of  heart;  SO,  somato- 
pleure;  SP,  splanchnopleure; 
TA,  allantois. 


KMI'.KYu    <»|'    SIA'KN    SKCMKNTS 


51 


this  region,  instead  a  diffuse  network  of  mesoderm  partly  fills  the  space  between 
ectoderm,  entoderm  and  mesothclium.  This  is  termed  mesenchyma  and  will 
be  described  later. 

Neural  plate  Neural  groove 

Ectoderm  \  /  .Mesodermal  segment 

Oomatic  mesoderm  \  jfex  'iS^.     /  Coelo 


Splanchnic  mesoderm  \  Notochord 

Descending  aorta  '  Entoderm 

Fig.  33. — Transverse  section  through  the  fifth  pair  of  mesodermal  segments  of  a  twenty-five-hour  chick 

embryo.     X  90. 


Neurci  enst 


Fig.  34. — Transverse  section  caudal  to  the  fovea  cardiaca  of  a  twenty-five-hour  chick  embryo.      X  90. 


Neural  tube 

Deseending  aorta 
Notochord 


Oplanchnic  mesoderm 
(Myocardium) 


Coelo, 


Ectoderm 
O0/71.  mesoderm 


Splanchnic  mesoderm  // 


Endothelium  of 
heart  tube 


Entoderm 


FlG.  35. — Transverse  section  through  the  fovea  cardiaca  of  a  twenty-five-hour  chick  embryo.     X  90. 


Transverse  Section  through  the  Fovea  Cardiaca  (Fig.  35).— This  section  is 
marked  by  a  vertical  layer  of  the  entoderm  at  the  point  where  it  is  folded  into 
the  head  as  the  fore-gut.  The  entoderm  is  thickened  laterally  and  forms  a 
continuous  mass  of  tissue  between  the  vitelline  veins.     The  splanchnic  mesoderm 


52 


THE    STUDY    OF    CHICK   EMBRYOS 


is  differentiated  into  a  thick  walled  pouch  on  each  side  lateral  to  the  endothelial 
layer  of  the  veins. 

Transverse  Section  through  the  Heart  (Fig.  36). — As  we  pass  cephalad  in 
the  series  of  sections  the  vitelline  veins  open  into  the  heart  just  in  front  of  the 
fovea  cardiaca.  The  entoderm  in  the  head-fold  now  forms  the  crescentic  pharynx 
or  fore-gut  separated  by  the  heart  and  splanchnic  mesothelium  from  the  entoderm 
of  the  germinal  disc.  The  descending  aortas  are  larger,  forming  conspicuous 
spaces  between  the  neural  tube  (hind-brain)  and  the  pharynx.  The  heart,  as 
will  be  seen,  is  formed  by  the  union  of  two  endothelial  tubes,  similar  to  those  which 
form  the  walls  of  the  vitelline  veins  in  the  preceding  sections.  The  median  walls 
of  these  tubes  disappear  at  a  slightly  later  stage  to  form  a  single  tube.     Thick- 


Ectoderrr, 

Somatic  mesoder 
Notochord 

Myocardium 
Entoderm 


■Neural  tube 

Descending  aorta 
Pharynx. 

Endocardium 
Splanchnic  rnes. 


Fig.  36. — Transverse  section  through  the  heart  of  a  twenty-five-hour  chick  embryo.     X  90. 

ened  layers  of  splanchnic  mesoderm  which,  in  the  preceding  section,  invested  the 
vitelline  veins  laterally,  now  form  the  mesothelial  wall  of  the  heart.  In  the 
median  ventral  line,  the  layers  of  splanchnic  mesoderm  of  each  side  have  fused, 
separated  from  the  splanchnic  mesothelium  of  the  germinal  disc  and  thus  the  two 
pleuro-pericardial  cavities  are  in  communication.  The  mesothelial  wall  of  the 
heart  forms  the  myocardium  and  epicardium  of  the  adult.  Dorsally,  the  splanch- 
nic mesoderm  is  continuous  with  the  somatic  mesoderm  and  forms  the  dorsal 
mesocardium. 


Origin  of  Primitive  Heart. — From  the  two  sections  just  described,  it  is  seen  that 
the  heart  arises  as  a  pair  of  endothelial  tubes  lying  in  the  pockets  of  the  splanchnic  mesoderm. 
Later,  the  endothelial  tubes  fuse  to  form  a  single  tube.  The  heart  then  consists  of  an  endo- 
thelial tube  within  a  thick-walled  tube  of  mesoderm.  The  origin  of  the  endothelial  cells  of  the 
heart  is  not  surely  known.  They  may  be  split  from  the  entoderm,  or  arise  from  the  mesoderm. 
According  to  another  view,  the  endothelium  arises  in  the  vascular  area  and  grows  into  the  body 


EMBRYO    OF    SKVKN    SKOMKMS 


53 


of  the  embryo.    The  vascular  system  is  primitively  a  paired  system,  the  heart  arising  as  a 
double  tube  with  two  veins  entering  and  two  arteries  leaving  it. 

Origin  of  the  Blood-vessels  and  Blood.  We  have  Been  that  in  the  area  opaca  a 
network  of  blood-vessels  and  blood-islands  are  differentiated  as  the  angioblast.  This  tissue 
gives  rise  to  all  of  the  primitive  blood-vessels  and  blood-cells  and  probably  is  derived  from  the 
splanchnic  mesoderm.  The  vessels  arise  first  as  reticular  masses  of  cells,  the  so-called  blood- 
islands.  These  cellular  thickenings  undergo  differentiation  into  two  cell  types,  the  innermost 
becoming  blood-cells,  the  outermost  forming  a  flattened  endothelial  layer  which  encloses  the 
blood-cells.  All  the  primitive  blood-vessels  of  the  embryo  are  composed  of  an  endothelial  layer 
only.  The  endothelial  cells  continue  to  divide,  forming  vascular  sprouts  and  in  this  way  new 
vessels  are  produced.  The  first  vessels  arising  in  the  vascular  area  of  a  chick  embryo  form  a 
close  network,  some  of  the  branches  of  which  enlarge  to  form  vascular  trunks.  One  pair  of 
such  trunks,  the  vitelline  veins,  is  differentiated  opposite,  and  later  connects  with,  the  posterior 
end  of  the  heart.  Another  pair,  the  vitelline  arteries,  are  developed  in  connection  with  the  aortae 
of  the  embryo.  The  vessels  of  the  vascular  area  thus  appear  before  those  of  the  embryo  have 
developed,  probably  arise  from  the  splanchnic  mesoderm,  and,  both  arteries  and  veins,  are  com- 
posed of  a  simple  endothelial  wall.  As  the  ccelom  develops  in  the  region  of  the  vascular  area 
of  the  embryo  soon  after  the  differentiation  of  the  angioblast  the  anlages  of  the  blood-vessels  are 
formed  only  in  the  splanchnic  layer.  (For  the  development  of  the  heart  and  blood-vessels 
see  Chapter  IX.) 

ectoderm 


Afesenclyma 

Moto  chord . 

Pharynqeal 
membrane— 

Entoderm      '^p-^'^^^"**'55^  ""^^r^g^ 

of  proamnion  XT  ^*  Bag—*"'''!!!—  .»i'Aji'  ' 

\S 

Fig.  37. — Transverse  section  through  the  pharyngeal  membrane  of  a  twenty-five-hour  chick  embryo. 

X  90. 

Transverse  Section  through  the  Pharyngeal  Membrane  (Fig.  37). — This 
section  passes  through  the  head-fold  and  shows  the  head  free  from  the  underlying 
germinal  area.  The  ectoderm  surrounds  the  head  and  near  the  mid-ventral  line 
is  bent  dorsad,  somewhat  thickened,  and  in  contact  with  the  thick  entoderm  of 
the  pharynx.  The  area  of  contact  between  ectoderm  and  pharyngeal  entoderm 
forms  the  pharyngeal  plate  or  membrane.  Later,  this  membrane  breaks  through 
and  thus  the  oral  cavity  arises.  The  expanded  neural  tube  is  closed  in  this 
region  and  forms  the  middle  brain  vesicle  or  mid-brain.  The  dorsal  aortae  appear 
as  small  vessels  dorsal  to  the  lateral  folds  of  the  pharynx.  The  germinal  area  in 
the  region  beneath  the  head  is  composed  of  ectoderm  and  entoderm  only.  This 
is  the  proamniotic  area.     Laterad  may  be  seen  the  layers  of  the  mesoderm. 


ryrrgea/ 
membrane— 


tj4  THE    STUDY    OF    CHICK   EMBRYOS 

Transverse  Section  through  the  Fore-brain  and  Optic  Vesicle  (Fig.  38). — 
The  neural  tube  is  open  here  and  constitutes  the  first  brain  vesicle  or  fore-brain. 
The  opening  is  the  anterior  neuropore.  The  ectoderm  is  composed  of  two  or  three 
layers  of  nuclei  and  is  continuous  with  the  much  thicker  wall  of  the  fore-brain. 
The  lateral  expansions  of  the  fore-brain  are  the  optic  vesicles,  which  eventually 
give  rise  to  the  retina  of  the  eye.     The  two  ectodermal  layers  are  in  contact 


Ectoderm 
Optic  vesicle ^ V?»w  '^ 


Neuropore 
Neural  tube 


"roamrtion 


<4&.  Pr 


Fig.  38. — Transverse  section  through  the  fore-brain  and  optic  vesicles  of  a  twenty-five-hour  chick.     X  90. 

with  each  other  except  in  the  mid-ventral  region,  where  the  mesenchyma  is 
beginning  to  penetrate  between  and  separate  them.  The  proamnion  consists 
of  a  layer  of  ectoderm  and  of  entoderm. 


CHICK  EMBRYO  OF  EIGHTEEN  PRIMITIVE  SEGMENTS  (THIRTY-SIX  HOURS) 

The  long  axis  of  this  embryo  is  nearly  straight  (Fig.  39),  the  area  pellucida 
is  dumb-bell  shaped  and  the  vascular  network  is  well  differentiated  throughout 
the  area  opaca.  The  tubular  heart  is  bent  to  the  right,  and  opposite  its  posterior 
end  the  vascular  network  converges  and  becomes  continuous  with  the  trunks  of 
the  vitelline  veins.  Connections  have  also  been  formed  between  the  descending 
aortae  and  the  vascular  area,  but  as  yet  the  vitelline  arteries  have  not  appeared 
as  distinct  trunks.  The  proamniotic  area  is  reduced  to  a  small  region  in  front  of 
the  head,  which  latter  is  now  larger  and  more  prominent.  In  the  posterior  third 
of  the  vascular  area  blood-islands  are  still  prominent. 

Central  Nervous  System  and  Sense  Organs. — The  neural  tube  is  closed  save 
at  the  caudal  end  where  the  open  neural  folds  form  the  rhomboidal  sinus.  In 
the  head  the  neural  tube  is  differentiated  into  the  three  brain  vesicles  marked  off 
from  each  other  by  constrictions.  The  fore-brain  (prosencephalon)  is  charac- 
terized by  the  outgrowing  optic  vesicles.  The  mid-brain  (mesencephalon)  is 
undifferentiated.  The  hind-brain  (rhombencephalon)  is  elongated  and  gradually 
merges  caudally  with  the  spinal  cord.  It  shows  a  number  of  secondary  constric- 
tions, the  neuromeres.     The  ectoderm  is  thickened  laterally  over  the  optic  ves- 


CHICK   EMBRYO   OF   EIGHTEEN   PRIMITIVE    SEGMENTS 


55 


icles  to  form  the  lens  placode  of  the  eye  (Fig.  41).  The  optic  vesicle  is  flattened 
at  this  point  and  will  soon  invaginate  to  produce  the  inner,  nervous  layer  of  the 
retina.  In  the  hind-brain  region,  dorso-laterally  the  ectoderm  is  thickened  and 
invaginated  as  the  auditory  placode  (Fig.  43).  This  placode  later  forms  the 
otocyst  or  otic  vesicle  from  which  is  differentiated  the  epithelium  of  the  internal 
ear  (membranous  labyrinth). 


Forsbram 
Mid-brain  \ 
Hind -bra  in 

Vitelline  Vein  _^ 


Mesodermal', 
segment  8 


Notochord  ~± 


Proamnion 
Optic  vesicle 

Free  portion 
of  he-ad 

Heart 


Neural  tube 


Bhomboidal 
sinus 


Primitive 
streaK 


Fig.  39. — View  of  the  dorsal  surface  of  a  thirty-six-hour  chick  embryo.     X  20. 


Digestive  Tube. — The  entoderm  is  still  flattened  out  over  the  surface  of  the 
yolk  caudal  to  the  fovea  cardiaca.  In  Fig.  40  the  greater  part  of  the  entoderm 
is  cut  away.  The  flattened  fore-gut,  folded  inward  at  the  fovea,  shows  indications 
of  three  lateral  diverticula,  the  pharyngeal  pouches.  Cephalad  the  pharynx  is 
closed  ventrally  by  the  pharyngeal  membrane. 


56 


THE    STUDY    OF    CHICK   EMBRYOS 


Heart  and  Blood-vessels. — As  seen  in  the  dorsal  view  of  the  embryo,  the 
heart  tube  is  bent  to  the  right.  Viewed  from  the  ventral  side,  the  bend  is  to  the 
left  (right  of  embryo)  (Fig.  40).  After  receiving  the  vitelline  veins  cephalad  to 
the  fovea  cardiaca  the  double-walled  tube  of  the  heart  dilates  and  bends  ventrad 


Optic  vesicle- 


Paired  ventral  aorta' 

Ventral  aorta 
Bulbus  cordis 

Ventricle 

Splanchnic  mesoderm 
Fovea  cardiaca 

R.  descending  aorta      K2T]|*~1 

Vascular  plexus — vyf  I 'a 

Splanchnic  mesoderm 


Xotochord 


\ 

/      [• 

\ 

Mes.  segment 


Segmental  zone 


Fore-brain 

Phar.  pouch  I 
Descending  aorta 

Phar.  pouch  II 
Somatopleure 

Left  vitelline  vein 
Entoderm 

Section  medullary  tube 

Somatopleure 
Descending  aorta 

Medullary  tube 
Splanchnic  mesoderm 

Capillary  plexus 
Somatopleure 


Neural  groove 

In..  40. — Ventral  reconstruction  of  a  thirty-six-hour  chick  embryo.     The  entoderm  has  been  removed 
save  about  and  caudal  to  the  fovea  cardiaca.     X  38. 


and  to  the  right  (left  of  Fig.  43).  It  then  is  flexed  dorsad  and  to  the  median  line, 
and  narrows  to  form  the  ventral  aorta.  The  aorta  lies  ventrad  to  the  pharynx 
and  divides  at  the  boundary  line  between  the  mid-  and  hind-brain  into  two 
ventral  aorta.     These  diverge  and  course  dorsad  around  the  pharynx.     Before 


CHICK   EMBRYO    OF   EIGHTEEN   PRIMITIVE    SEGMENTS 


57 


reaching  the  optic  vesicles  they  bend  caudad,  and  as  the  paired  descending  aortae 
may  be  traced  to  a  point  opposite  the  last  primitive  segments.  In  the  region  of 
the  fovea  cardiaca  they  lie  close  together  and  have  fused  to  form  a  single  vessel, 
the  dorsal  aorta.  They  soon  separate  and  opposite  the  last  primitive  segments 
they  are  connected  by  numerous  capillaries  with  the  vascular  network.  In  this 
region  at  a  later  stage  the  trunks  of  the  paired  vitelline  arteries  will  be  differen- 
tiated. The  heart  beats  at  this  stage,  the  blood  flows  from  the  vascular  area  by 
way  of  the  vitelline  veins  to  the  heart,  thence  by  the  aortae  and  vitelline  arteries 
back  again.  This  constitutes  the  vitelline  circulation  and  through  it  the  embryo 
receives  nutriment  from  the  yolk  for  its  future  development. 


Lens  an/aye 


Fig.  41. — Transverse  section  through  the  fore-brain  of  a  thirty-six-hour  chick  embryo.     X  75. 


In  studying  transverse  sections  of  the  embryo  the  student  should  not  only 
identify  the  structures  seen,  but  should  locate  the  level  of  each  section  by  compar- 
ing with  Figs.  39  and  40,  and  trace  the  organs  from  section  to  section  in  the  series. 

Transverse  Section  through  the  Fore-brain  and  Optic  Vesicles.  (Fig.  41). — 
The  optic  stalks  connect  the  optic  vesicles  laterally  with  the  ventral  portion  of  the  fore- 
brain.  Dorsally  the  section  passes  through  the  mid-brain.  We  have  alluded  to  the  thickening 
of  the  lens  placode.  Note  that  there  is  now  a  considerable  amount  of  mesenchyma  between  the 
ectoderm  and  the  neural  tube.     In  the  germinal  area  the  layers  of  mesoderm  are  present. 

Transverse  Section  through  the  Pharyngeal  Membrane  and  Mid-brain  (Fig. 
42). — In  the  mid-ventral  line  the  thickened  ectoderm  bends  up  into  contact  with  the  entoderm 
of  the  rounded  pharynx.  At  this  point  the  oral  opening  will  break  through.  On  either  side 
of  the  pharynx  a  pair  of  large  vessels  are  seen;  the  ventral  pair  are  the  ventral  aorta-.  Two 
sections  cephalad  their  cavities  open  into  those  of  the  dorsal  pair,  the  descending  aorta:.  The 
section  is  thus  just  caudad  to  the  point  where  the  ventral  aorta?  bend  dorsad  and  caudad  to 
form  the  descending  aorta?.  The  section  passes  through  the  caudal  end  of  the  mesencephalon 
which  is  here  thick  walled  with  an  oval  cavity.     Note  the  large  amount  of  undifferentiated 


58 


THE   STUDY   OF   CHICK   EMBRYOS 


mesenchyma  in  the  section.     The  structure  of  the  germinal  area  is  complicated  by  the  presence 
of  collapsed  blood-vessels. 

Transverse  Section  through  the  Hind-brain  and  Auditory  Placodes  (Fig. 
43). — Besides  the  auditory  placodes  already  described  as  the  anlages  of  the  internal  ear,  this 
section  is  characterized  (1)  by  the  large  hind-brain,  somewhat  flattened  dorsad;  (2)  by  the 
broad  dorso-ventrahy  flattened  pharynx,  above  which  on  each  side  he  the  dorsal  aortce;  (3)  by  the 


Ectoderm 


Mesenchyma 


Descending 
aorta 


Neural  tube 


Notochord 


Forequt 

Pharyngeal 
membrane 


Splanchnopleure 

Fig.  42. — Transverse  section  through  the  pharyngeal  membrane  of  a  thirty-six-hour  chick  embryo. 

X7S- 


Ectoderm 


Notochord 
Descending  Qorta 


Pericardial 
cavity 

Somatic  fne 


Neural  tube 

Ant  Cardinal  Vein 
Auditory  placode 

Forego.! 

Ectoderm 


Endothelium 
of  heart      vv 


toderm 

Endothelium  of  ventral  aorta 
Myocardium 


Fig.  43. — Transverse  section  through  the  hind-brain  and  auditory  placodes  of  a  thirty-six-hour  chick 

embryo.     X  75. 


presence  of  the  ventral  aorta  and  bulbar  portion  of  the  heart.  The  descending  aortae  are  located 
on  each  side  dorsal  to  the  pharynx.  The  ventral  aorta  is  suspended  dorsally  by  the  mesoderm, 
which  here  forms  the  dorsal  mesocardium.  The  bulbus  of  the  heart  lies  to  the  left  in  the  figure 
fright  of  embryo)  and  a  few  sections  caudad  in  the  series  is  continuous  with  the  ventral  aorta 
(see  Fig.  40).  Between  the  somatic  and  splanchnic  mesoderm  is  the  large  pericardial  cavity. 
It  surrounds  the  heart  in  this  section. 


CHICK    EMBRYO   OF    EIGHTEEN    PRIMITIVI     SEGMENTS 


59 


Transverse  Section  through  the  Caudal  End  of  the  Heart  (Fig.  44. )—  The 
section  passes  through  the  hind-brain.  The  descending  aorta-  are  separated  only  by  a  thin 
septum  which  is  ruptured  in  this  section.  The  mesothelial  wall  of  the  heart  is  continuous  with 
the  somatic  mesoderm.  On  the  right  side  of  the  section  there  is  apparent  fusion  between  the 
myocardium  of  the  heart  and  the  somatic  mesoderm.     Lateral  to  the  aorta."  are  the  anterior  cardinal 


Ectoder, 


Mes. 
Ant.  cardinal  Vein 


'eural  tube 

Foreou-V 


Splanchnic  mesoderm 


Entoderm 

Vitelline  vein  /       """> —  ^  Splanchnic  mesoderm 

Heart  I  tyocardium 

Fig.  44. — Transverse  section  through  the  caudal  end  of  the  heart  of  a  thirty-six-hour  chick  embryo. 

X75- 


sSomdtopleure 


Edoderr 
Mes  Segment 


Dorsal  aorta. 
Coelon 


Neural  tube 
chord 


Extra   embryonic 
coelom 

^Entoderm 

L.vit.  ve<n  I  \ Right  vitelline  van 

Spl  mesoderm  I 

Entoderr-  Foreau-t 

Fig.  45. — Transverse  section  through  the  fovea  cardiaca  of  a  thirty-six-hour  chick  embryo.  Ant. 
card,  vein,  anterior  cardinal  vein;  L.  vit.  vein,  left  vitelline  vein;  Mes.  segment,  mesodermal  segment; 
Spl.  mesoderm,  splanchnic  mesoderm.     X  90. 


veins.  A  pair  of  primitive  mesodermal  segments  may  be  seen  in  this  section  lateral  to  the  hind- 
brain.  It  may  be  noted  here  that  the  primitive  segments  were  not  present  in  the  sections  of 
the  head  previously  studied. 

Transverse  Section  through  the  Fovea  Cardiaca  (Fig.  45).— The  descending 
aorta  now  form  a  single  vessel,  the  dorsal  aorta,  the  medium  septum  having  disappeared.  The 
section  passes  through  the  entoderm  at  the  point  where  it  is  folded  dorsad  and  cephalad  into  the 


6o 


THE    STUDY    OF    CHICK   EMBRYOS 


head  as  the  fore-gut.  The  cavity  is  the  fovea  cardiaca  and  two  sections  caudad  it  communicates 
with  the  flattened  space  between  the  entoderm  and  the  yolk.  On  each  side  of  the  fore-gut  are 
the  large  vitelline  veins,  sectioned  obliquely.  As  the  splanchnic  mesoderm  overlies  these  veins 
dorsad,  it  is  pressed  by  them  on  each  side  against  the  somatic  mesoderm  and  the  cavity  of  the 
ccelom  is  thus  interrupted. 

Transverse  Section  Caudal  to  the  Fovea  Cardiaca  (Fig.  46). — This  section  re- 
sembles the  preceding  save  that  the  primitive  gut  is  without  a  ventral  wall.  The  vitelline  vein 
on  the  left  is  still  large. 

Neural  lube 


Mes.  Segment 


Neural  ccu/cty 

chderm 
Notbchord 


Dorsal  aorta 
•Somato  pleure 


^Xe^*  /  /  \  Entoderm 

Jplanchnopkure   Open    jut 
Fig.  46. — Transverse  section  caudal  to  the  fovea  cardiaca  of  a  thirty-six-hour  chick  embryo.     X  90. 


Mes. segment- 
Central  cells 
o  i segment 


Neural  tube 

Ectoderm 


Mesonephric  duct 


■Som.  mesoderm 


Splanchnic     . 
mesoderm 


Coelom 


Descending    .. 

aorta      notbchord 


Fig.  47. — Transverse  section  through  the  fourteenth  pair  of  mesodermal  segments  of  a  thirty-six-hour 

chick  embryo.      X  90. 


Section  through  the  Fourteenth  Pair  of  Primitive  Segments  (Fig.  47). — The 
body  of  the  embryo  is  now  flattened  on  the  surface  of  the  yolk.  The  dorsal  aortae  have  sepa- 
rated and  occupy  the  depressions  lateral  to  the  primitive  segments.  The  section  is  characterized 
by  the  differentiated  mesoderm  which  forms  the  primitive  segments,  nephrotomes,  somatic  and 
splanchnic  mesoderm,  structures  soon  to  be  described. 

Transverse  Section  through  the  Rhomboidal  Sinus  (Fig.  48). — The  neural  groove 
is  open,  the  notochord  is  oval  in  form.  The  ectoderm  is  characterized  by  the  columnar  form  of 
its  cells.  At  the  point  where  the  ectoderm  joins  the  neural  fold  a  crest  of  cells  projects  ventrally 
on  either  side.  These  projecting  cells  form  the  neural  crests,  and  from  them  the  spinal  ganglia 
are  formed.  The  mesodermal  plates  have  split  laterally  into  layers,  but  the  ccelomic  cavities 
are  mere  slits.     Between  the  splanchnic  mesoderm  and  the  entoderm  blood-vessels  may  be  seen. 


CHICK   EMBRYO    OF   EIGHTEEN   PRIMITIVE    SEGMENTS 


6l 


Transverse  Section  through  the  Primitive  (Hensen's)  Node  or  Knot  (Fig. 
49). — The  section  shows  the  three  germ  layers  bound  together  at  the  "knot"  or  node  into  a 
mass  of  undifferentiated  tissue.  The  mesoderm  is  split  laterally  into  the  somatic  and 
splanchnic  layers. 

Transverse  Section  through  the  Primitive  Streak  (Fig.  50). — In  the  mid-dorsal 
line  is  the  primitive  groove.     The  germ  layers  may  be  seen  taking  their  origin  from  the  undiffer- 


[ctoderm 


,  Neural  tube 

/Neural  Crest 


Segmental  Zone 


Somalic  mesoderm 


Splanchnic  mesoderm       ~    ,      I  a/4,    r      ,1  \  «r  *  1  ^  Blood  vessel 

'  Coelom'  /V0T0  chord'  \  entoderm 

Fig.  48. — Transverse  section  through  the  rhomboidal  sinus  of  a  thirty-six-hour  chick  embryo.     X  90. 


Ectoderm 


Somatic  tnesoi 


Primitive  node 


Coelot 

Entoderm  I  Splanchnic  mesoderm 

Fig.  49. — Transverse  section  through  the  primitive  (Hensen's)  node  of  a  thirty-six-hour  chick  embryo. 

X  90. 


Primitive  groove 
Ectoderm 


Sptanchnopteu  re 


Coelom 
Entoderm/  Primitive  streak         Splanchnic  mes- 

Fig.  50. — Transverse  section  through  the  primitive  streak  of  a  thirty-six-hour  chick  embryo.     X  90. 


entiated  tissue  of  the  primitive  streak  beneath  the  primitive  groove.     Between  the  splanchnic 
mesoderm  and  entoderm  blood-vessels  are  present  laterad  as  in  the  preceding  sections. 

Mesodermal  Segments. — We  have  seen  that  these  are  developed  by  the  ap- 
pearance of  transverse  furrows  in  the  mesoderm  (Fig.  51).  Later  a  longitudinal 
furrow  partially  separates  the  paired  segments  from  the  lateral  unsegmented 
mesoderm.     The  segments  are  block-like  with  rounded  angles  when  viewed 


62 


THE    STUDY   OP   CHICK   EMBRYOS 


dorsally,  triangular  in  transverse  section  (Figs.  47  and  51).  They  are  formed 
cranio-caudally,  the  most  cephalad  being  the  first  to  appear.  The  first  three  He 
in  the  head  region.  The  segments  contain  no  cavity  but  a  potential  cavity  repre- 
senting a  portion  of  the  ccelom  is  filled  with  cells,  and  the  other  cells  of  the  seg- 
ments form  a  thick  mesothelial  layer  about  them.  The  ventral  wall  and  a  por- 
tion of  the  median  wall  of  each  primitive  segment  become  transformed  into 
mesenchyma  which  surrounds  the  neural  tube  and  notochord  (Fig.  282).  The 
remaining  portion  of  the  segments  persist  as  the  dermo-muscular  plates  or  myo- 
tomes. The  cells  of  the  myotomes  elongate  and  give  rise  to  the  voluntary  muscle 
of  the  body.     The  voluntary  or  skeletal  muscles  are  thus  at  first  all  segmented 


Mesonephric  dad 


Neural  tube 


Mes.  segment 


Sovi.  mesoderm 
Splanchnopleure 
Descending  aorta 


Notochord        Entoderm 


Fig.  51. — Semi-cliagrammatic  reconstruction  of  five  mesodermal  segments  of  a  forty-eight-hour  chick 
embryo.     The  ectoderm  is  removed  from  the  dorsal  surface  of  the  embryo. 


but  later  many  of  the  segments  fuse.     In  the  trunk  muscles  of  the  adult  fish  the 
primitive  segmented  condition  is  retained. 

The  Intermediate  Cell  Masses  or  Nephrotomes. — The  bridge  of  cells  con- 
necting the  primitive  segments  with  the  lateral  mesodermal  layers  constitutes 
the  nephrotome  (Figs.  47  and  51).  The  nephrotomes  give  rise  dorsad  to  pairs 
of  small  cell  masses  segmentally  arranged  in  the  furrow  lateral  to  the  primitive 
segments.  By  the  union  of  these  cell  masses  solid  cords  are  formed  which  run 
lengthwise  in  the  furrow.  These  cords  hollow  out,  grow  caudad,  and  become 
the  primary  excretory  (mesonephric)  ducts  (Fig.  51).  The  rest  of  the  intermediate 
cell  mass  becomes  the  embryonic  kidney  or  mesonephros,  the  tubules  of  which  open 
into  the  primary  excretory  duct.     As  the  genital  glands  develop  in  connection 


CHICK   EMBRYO    OF   EIGHTEEN   PRIMITIVE    SEGMENTS 


63 


Notochord     n_ 

y  Neural   tube 

fileph  rotome     ^\ 

•  >/'      _^~-  A^es.  segment 

Archenkron    — 7^^£^S 

Splanchnic     — iff  tiff 
mesoderm        III  Ijff 

"9p»Si^§n?^^^— ^  Ectoderm 

^n&.    \^       Somatic 
\&.  ^*      mesoderm 

Entoderm  — 1|    jp 

|i vs^* 

1/ 

with  the  mesonephros,  and  as  the  kidney  of  the  adult  (metanephros)  is  partly 
developed  as  an  outgrowth  of  the  primary  excretory  duct,  we  may  regard  the 
intermediate  cell  mass  as  the  anlage  of  the  urogenital  glands  and  their  ducts.  They 
are  thus  of  mesodermal  origin. 

Somatopleure  and  Splanchnopleure. — In  the  embryo  of  seven  primitive  seg- 
ments we  saw  that  the  mesoderm  split  laterally  into  two  layers,  the  somatic 
(dorsal)  and  the  splanchnic  (ventral)  mesoderm.  These  layers  persist  in  the  adult, 
the  somatic  mesoderm  giving  rise  to  the  pericardium  of  the  heart,  to  the  parietal 
pleura  of  the  thorax  and  to  the  peritoneum  of  the  abdomen,  while  the  splanchnic 
layer  forms  the  epicardium  and  myocardium  of  the  heart,  visceral  pleura  of  the 
lungs,  the  mesenteries  and 
mesodermal  layer  of  the  gut. 
The  somatic  mesoderm  and 
the  ectoderm  with  the  tissue 
developed  between  them  con- 
stitute the  body  wall,  which  is 
termed  the  somatopleure.  In 
the  same  way  the  splanchnic 
mesoderm  and  the  entoderm 
with  the  mesenchymal  tissue 
between  them  constitute  the 
wall  of  the  gut,  termed  the 
splanchnopleure. 

Ccelom. — The  cavity  be- 
tween the  somatopleure  and 
splanchnopleure  is  the  ccelom 

(body  cavity).  With  the  splitting  of  the  mesoderm,  isolated  cavities  are 
produced.  These  unite  on  each  side  and  eventually  form  one  cavity — the 
ccelom.  With  the  extension  of  the  mesoderm,  the  ccelom  surrounds  the  heart 
and  gut  ventrally  (Fig.  52).  Later,  it  is  subdivided  into  the  pericardial  cavity 
of  the  heart,  the  pleural  cavity  of  the  thorax  and  the  peritoneal  cavity  of  the 
abdominal  region.  In  the  stages  we  have  studied,  the  embryo  is  flattened  on  the 
surface  of  the  yolk  and  the  somatopleure  and  splanchnopleure  do  not  meet  ven- 
trad.  If  this  were  the  case  we  should  have  the  structural  relations  as  in  Fig.  52 
which  is  essentially  the  ground  plan  of  the  vertebrate  body. 

Mesenchyma. — In  the  sections  through  the  head  of  this  embryo  and  through 
that  of  the  preceding  stage,  we  have  found  but  three  primitive  segments  present. 
The  greater  part  of  the  mesoderm  in  the  head  appears  in  the  form  of  an  undif- 


Fig.  52. 


\Coelom 

-Diagrammatic  transverse  section  of  a  vertebrate 
embryo  (adapted  from  Minot). 


64 


THE    STUDY    OF    CHICK   EMBRYOS 


ferentiated  network  of  cells  which  fill  in  the  spaces  between  the  definite  layers 
(epithelia).  This  tissue  is  mesenchyma  (Fig.  $$).  The  mesoderm  may  be  largely 
converted  into  mesenchyma  as  in  the  head,  or  any  of  the  mesodermal  layers 
may  contribute  to  its  formation.  Thus  it  may  be  derived  from  the  primitive 
segments  and  from  the  somatic  and  splanchnic  mesoderm.  The  cells  of  the 
mesenchyma  form  a  syncytium  or  network,  and  are  at  first  packed  closely  to- 
gether. Later,  they  may  form  a  more  open  network  with  cytoplasmic  processes 
extending  from  cell  to  cell  (Fig.  53).  The  mesenchyma  is  an  important  tissue  of 
the  embryo,  as  from  it  are  differentiated  the  blood  and  lymphatic  systems, 

together  with  most  of  the  smooth  muscle, 
connective  tissue,  and  skeletal  tissue  of 
the  body. 

The  body  of  the  embryo  is  now  com- 
posed (1)  of  cells  arranged  in  layers — epi- 
thelia, and  (2)  of  diffuse  mesenchyma.  The 
term  "epithelium"  may  be  used  in  a 
general  sense  or  restricted  to  layers  cover- 
ing the  surface  of  the  body  or  lining  the 
digestive  canal  and  its  derivatives.  Layers 
lining  the  body  cavities  are  termed  meso- 
thelia,  while  those  lining  the  blood-vessels 
and  heart  are  called  endothelia. 
Derivatives  of  the  Germ  Layers. — The  tissues  of  the  adult  are  derived  from 
the  epithelia  and  mesenchyma  of  the  three  germ  layers  as  follows: 


Ectoderm 

Mesen- 
chyma 


Fig.  53. — Mesenchyma  from  the  head  of  a 
thirty-si.\-hour  chick  embryo.     X  495. 


Ectoderm 

Mesoderm 

Entoderm 

I. 

Epidermis   and   its   deriva- 

A 

Mesothelium. 

1. 

Epithelium   of   digestive 

tives  (hair,  nails,  glands). 

1. 

Pericardium. 

tract. 

2. 

Conjunctiva  and  lens  of  eye. 

2. 

Pleura. 

2. 

Liver. 

3- 

Sensorj'  epithelia  of  organs 

3- 

Peritoneum. 

3- 

Pancreas. 

of  special  sense. 

4- 

Serous  layer  of  intestine. 

4- 

Epithelium  of  pharynx. 

4- 

Epithelium  of  mouth,  enam- 
el   of     teeth,    oral    glands. 

5- 

Epithelium   of   uro- 
gans. 

genital   or- 

Eustachian  tube. 
Tonsils. 

Pituitary  body. 

6. 

Striated  muscle. 

Thymus. 

5- 

Epithelium  of  anus. 

1.  Skeletal. 

Thyreoids. 

6. 

Epithelium  of  amnion  and 

2.  Cardiac. 

Epithelial  bodies.    |  OaA^V.  -oa^ 
Epithelium  of  respiratory 

chorion. 

B 

Mesenchyma. 

5- 

7- 

Nervous,      neuroglia      and 

1. 

Blood-cells. 

tract. 

chromaffin  cells  of  nervous 

2. 

Bone  marrow. 

Larynx. 

system.     Retina  and   optic 

3- 

Endothelium  of  blood-vessels. 

Trachea. 

nerve. 

4- 

Endothelium  of  lymphatics  and 

Lungs. 

8. 

Xotorhord  (?) 

spleen. 

6. 

Notochord  (?) 

9- 

Smooth    muscle    of     sweat 

5- 

Supporting  tissues. 

(Connect- 

glands  and  of  iris. 

ing  tissue,  cartilage  and  bone.) 

6. 

Smooth  muscle. 

CHICK   EMBRYO    OF   TWENTY-SEVEN    SEGMENTS 


65 


For  the  histological  development  (histogenesis)  of  the  various  tissues  from 
the  primary  germ  layers  see  Chapter  X. 


CHICK  EMBRYO  OF  TWENTY-SEVEN  SEGMENTS  (FIFTY  HOURS) 

This  embryo,  which  is  taken  as  a  type  of  the  forty-eight  to  fifty-two-hour 

stage,  lies  in  the  center  of  the  vascular  area  and  is  peculiar  in  that  the  head  is 

twisted  qo°  to  the  right.     We  therefore  see  the  right  side  of  the  head  but  the  dorsal 

side  of  the  body.     In  the  region  of  the  mid-brain  is  the  very  marked  head-bend 


mm 


Hind-brain 


Otic  vesicle  * 


Branchial  clefts 
Amnion  fold. 


M 


num     w 


•^   Midbrain 


Ap-v-'Ni  Fort-brain 

^  n  t-  ■  i 

-•■  \  v^f'S  Optic  vesicle 

P^P^  Lens  vesicle 


Ventricle  of  heart 
Vitelline  vein 


I.  Vitelline  artery 
Neural  tube 


AH 

Primitive  streak  (_fj^.i  r  A  Vl 
Fig.  54. — Dorsal  view  of  a  fifty-hour  chick  embryo,  stained  and  mounted  in  balsam.     X  14. 


or  cephalic  flexure.  Below  the  head  and  ventral  in  position  lies  the  tubular  heart, 
now  bent  in  the  form  of  a  letter  S.  Dorsal  to  the  heart  in  the  region  of  the 
pharynx,  three  transverse  grooves  or  slits  may  be  seen.  These  are  the  branchial 
clefts  or  gill  slits.  The  head  of  the  embryo  is  now  covered  by  a  double  fold  of 
the  somatopleure,  the  head-fold  of  the  amnion.     It  envelops  the  head  like  a  veil. 


66 


THE    STUDY   OF   CHICK  EMBRYOS 


Caudally  a  fold  and  opacity  mark  the  position  of  the  tail-fold  from  which  de- 
velops the  caudal  end  of  the  body.  The  curved  fold  embracing  this  is  the  tail- 
fold  of  the  amnion  which  will  eventually  meet  the  head-fold  and  completely 
enclose  the  embryo. 


Mid-brain 


Optic 
Aperture  of  lens  n 

Fore-brai 

Phary 

Bulb  of  heart 

Ventricle 

R.  vitelline  vein 

Fore-gut 

Splanchnopleure 
Splanchnic  mesoderm 

Dorsal  aorta 
R.  vitelline  artery 

Mes.  segment 

Segmental  zone 

Neural  plate 

Entoderm 
Primitive  node 


nd-brain 

Notochord 

Otocyst 


Aortic  arches  i,  2,  3 
Ant.  cardinal  vein 
Atrium 

Common  cardinal  vein 
Post  cardinal  vein 
Descending  aorta 
Liver  anlage 
Fovea  cardiaca 


Entoderm 
Somatopleure 
Spinal  cord 

L.  vitelline  artery 


Edge  of  splanchnic  mesoderm 
Mes.  segment 

Vascular  plexus 


Notochord 


Hind-gut 


FlG.  55. — Semi-diagrammatic  reconstruction  of  a  fifty-hour  chick  embryo,  ventral  view.  X  22. 
The  entoderm  has  been  removed  save  in  the  region  of  the  fovea  cardiaca  and  of  the  hind-gut.  The 
cranial  third  of  the  embryo  is  seen  from  the  left  side,  the  caudal  two-thirds  in  ventral  view  owing  to  the 
torsion  of  the  embryo. 


Central  Nervous  System  and  Sense  Organs  (Fig.  55). — The  neural  tube  is 
divided  by  constrictions  cephalad  into  four  vesicles.  The  fore-brain  of  the 
previous  stage  is  now  subdivided  into  two  regions,  the  telencephalon  and  dien- 
cephalon.  The  cephalic  flexure  has  been  established  in  the  region  of  the  mesen- 
cephalon.    The  hind-brain  is  as  yet  undivided  and  is  as  long  as  the  other  three 


CHICK   EMBRYO   OF   TWENTY-SEVEN   SEGMENTS  67 

vesicles.  The  lens  of  the  eye  has  invaginated,  pushing  in  the  wall  of  the  optic 
vesicle  and  thus  forming  a  double-walled  structure,  the  optic  cup.  The  audi- 
tory placode  has  become  a  sac,  the  otocyst,  which  overlies  the  hind-brain  opposite 
the  second  branchial  groove  and  is  still  connected  with  the  outer  ectoderm,  cut 
away  in  Fig.  55.  The  rhomboidal  sinus  is  still  open  at  the  caudal  end  of  the 
neural  tube  (Fig.  54). 

Digestive  Canal. — In  a  reconstruction  from  the  ventral  side  the  digestive 
canal  shows  differentiation  into  three  regions.  Of  these,  the  fore-gut  we  have 
seen  in  earlier  stages;  the  mid-gut  is  without  ventral  wall  and  overlies  the  yolk. 
A  greater  part  of  the  mid-gut  has  been  cut  away  to  show  the  underlying  struc- 
tures. Caudad,  a  small  fovea  leads  into  the  hind-gut  which  is  just  beginning  to 
evaginate  into  the  tail-fold.  The  pharyngeal  membrane  now  lies  in  a  consider- 
able cavity,  the  stomodceum,  formed  by  the  invaginated  ectoderm.  The  median 
ectodermal  pouch  next  the  brain-wall  is  known  as  Rathke's  pocket  and  is  the  an- 
lage  of  the  anterior  lobe  of  the  hypophysis.  The  pharynx  shows  laterally  three 
out  pocketings,  of  which  the  first  is  wing  like  and  is  the  largest.  These  pharyngeal 
pouches  occur  opposite  the  three  branchial  grooves  and  at  these  points  entoderm 
and  ectoderm  are  in  contact.  Between  them  are  developed  the  branchial  arches, 
in  which  course  the  paired  aortic  arches.  Towards  the  fovea  cardiaca  the  fore-gut 
is  flattened  laterally  and  before  it  opens  out  into  the  mid-gut  there  is  budded  off 
ventrally  a  bilobed  structure,  the  anlage  of  the  liver  (Fig.  60).  It  lies  between  the 
vitelline  veins  and  in  its  later  development  the  veins  are  broken  up  into  the 
sinusoids  or  blood  spaces  of  the  liver. 

Just  as  the  entoderm  grows  out  into  the  head-fold  to  form  the  fore-gut  so 
it  grows  into  the  tail-fold  and  forms  the  hind-gut.  This  at  once  gives  rise  to  a 
tubular  outgrowth  which  becomes  the  allantois,  one  of  the  fetal  membranes  to 
be  described  later. 

Blood  Vascular  System. — The  tubular  heart  is  flexed  in  the  form  of  a  letter 
S  when  seen  from  the  ventral  side.  Four  regions  may  be  distinguished:  (1) 
The  sinus  venosus,  into  which  open  the  veins;  (2)  a  dilated  dorsal  chamber,  the 
atrium;  (3)  a  tubular  ventral  portion  flexed  in  the  form  of  a  U,  of  which  the  left 
limb  is  the  ventricle,  the  right  limb  (4)  the  bulbus  cordis.  From  the  bulbus  is 
given  off  the  ventral  aorta.  There  are  now  developed  three  pairs  of  aortic  arches 
which  open  into  the  paired  descending  aortae.  The  first  aortic  arch  passes 
cranial  to  the  first  pharyngeal  pouch  and  is  the  primitive  arch  seen  in  the 
thirty-six-hour  embryo.  The  second  and  third  arches  course  on  either  side 
of  the  second  pharyngeal  pouch.     They  are  developed  by   the  enlargement 


68 


THE    STUDY   OF   CHICK   EMBRYOS 


of  channels  in  primitive  capillary  networks  between  ventral  and  descending 
aortae.  Opposite  the  sinus  venosus  the  paired  aortic  trunks  fuse  to  form  the  single 
dorsal  aorta  which  extends  as  far  back  as  the  fifteenth  pair  of  primitive  segments. 
At  this  point  the  aortae  again  separate  and  opposite  the  twentieth  segments  each 
connects  with  the  trunk  of  a  vitelline  artery  which  was  developed  in,  and  conveys 
the  blood  to,  the  vascular  area  (Fig.  55).  Caudal  to  the  vitelline  arteries  the 
dorsal  aortae  rapidly  decrease  in  size  and  soon  end. 


Hind-brain 


Notochord 


Ectoder 


Lens  vesicle 


Cavity  of . 
forebrcvn 


Ant.CardinaJ 
vein 


Aortic  arch  I 


Optic  vesicle 
Prosencephalon 


Fig.    56. — Transverse  section  through  the  fore-brain  and  eyes  of  a  fifty-hour  chick  embryo.     X  50. 


As  in  the  previous  stage,  the  blood  is  conveyed  from  the  vascular  area  to  the 
heart  by  the  vitelline  veins,  now  two  large  trunks.  In  the  body  of  the  embryo 
there  have  developed  two  pairs  of  veins.  In  the  head  have  appeared  the  anterior 
cardinal  veins,  already  of  large  size  and  lying  lateral  to  the  ventral  region  of  the 
brain  vesicles  (Fig.  58).  Caudal  to  the  atrium  of  the  heart,  two  smaller  posterior 
cardinal  veins  are  developed.  They  lie  in  the  mesenchyma  of  the  somatopleure 
laterad  in  position  (Fig.  60).  Opposite  the  sinus  venosus  the  anterior  and  pos- 
terior cardinal  veins  of  each  side  unite  and  form  the  common  cardinal  veins  (ducts 


CHICK   EMBRYO   OF   TWENTY-SEVEN    SEGMENTS 


69 


of  Cuvier)  which  open  into  the  dorsal  wall  of  the  sinus  venosus.  The  primitive 
veins  are  thus  paired  like  the  arteries,  and  like  them  develop  by  the  enlargement 
of  channels  in  a  network  of  capillaries. 

The  following  series  of  transverse  sections  from  an  embryo  of  this  stage  shows 
the  more  important  structures.  The  approximate  plane  and  level  of  each  section 
may  be  seen  by  referring  to  Figs.  54  and  55. 


Blastoderm 
Hind-brain 


Mesenchy, 


Amnion 
Chorion 


Fig.  57. — Transverse  section  through  the  optic  stalks  and  hypophysis  of  a  fifty-hour  chick  embryo. 

X50. 


Section  through  the  Fore-brain  and  Eyes  (Fig.  56). — The  section  passes  cranial 
to  the  optic  stalks,  consequently  the  optic  vesicles  appear  unconnected  with  the  fore-brain. 
The  thickened  ectoderm  is  invaginated  to  form  the  anlages  of  the  lens  vesicles.  The  thicker 
wall  of  the  optic  vesicles  next  the  lens  anlage  will  give  rise  to  the  nervous  layer  of  the  retina,  the 
thinner  outer  wall  becomes  the  pigment  layer  of  the  retina.  Ventrad  in  the  section  are  the  wall 
and  cavity  of  the  fore-brain,  dorsad  the  hind-brain  with  its  thin  dorsal  ependymal  layer.  Be- 
tween the  brain  vesicles  on  either  side  are  sections  of  the  first  aortic  arches  and  lateral  to  the  hind- 
brain  are  the  smaller  paired  anterior  cardinal  veins,  which  convey  the  blood  from  the  head  to 
the  heart. 

Section  through  the  Optic  Stalks  and  Hypophysis  (Fig.  57). — The  section  passes 
just  caudal  to  the  lens  which  does  not  show.  The  optic  vesicles  are  connected  with  the  wall  of 
the  fore-bra  in  by  the  optic  stalks  which  later  form  the  path  by  which  the  fibers  of  the  optic  nerve 
pass  from  the  retina  to  the  brain.     Both  the  ventral  and  the  descending  aorta:  are  seen  in  section 


7° 


THE    STUDY   OF   CHICK  EMBRYOS 


about  the  cephalad  end  of  the  pharynx.     Between  the  ventral  wall  of  the  fore-brain  and  the 
pharynx  is  an  invagination  of  the  ectoderm,  Rathke's  pocket. 

Section  through  the  Otocysts  and  Second  Aortic  Arch  (Fig.  58). — The  otic 
vesicles  are  sectioned  caudal  to  their  apertures  and  appear  as  closed  sacs  lateral  to  the  wall  of 
the  hind-brain.  The  cavity  of  the  pharynx  is  somewhat  triangular  and  its  dorsad  wall  is  thin. 
The  anterior  cardinal  veins  pass  between  the  otocysts  and  the  wall  of  the  hind-brain.  Ventral 
to  the  pharynx  the  bulbus  cordis  is  sectioned  obliquely  where  it  leaves  the  heart,  and  at  this 
level  gives  off  laterad  the  second  pair  of  aortic  arches  which  connect  dorsad  with  the  descending 
aorta?.  Surrounding  the  bulbus  cordis  is  the  large  pericardial  cavity.  Between  the  first  and 
second  aortic  arches  (Fig.  58)  is  the  first  pair  of  pharyngeal  pouches,  lateral  diverticula  of  the 
entoderm.     The  student  should  note  that  in  the  sections  of  this  stage  so  far  studied,  the  tnesen- 


Aartitarch 

2 


Ventral 
aorta. 


Endothelium 
of  bulbus 


Fig.  58. — Transverse  section  through  the  otic  vesicles  and  second  aortic  arches  of  a  fifty-hour  chick 

embryo.     X  50. 


chyma  of  the  head  is  undifferentiated,  the  tissues  peculiar  to  the  adult  not  yet  having  been 
formed. 

Section  through  the  Sinus  Venosus  and  Common  Cardinal  Veins  (Fig.  59). — At 
this  level,  the  common  trunk  formed  by  the  anterior  and  posterior  cardinal  veins  opens  into 
the  thin- walled  sinus  venosus.  The  sinus  receives  all  of  the  blood  passing  to  the  heart  and  is 
separated  only  by  a  slight  constriction  from  the  larger  atrium.  The  muscle  plates  of  the  first 
mesoflermal  segments  are  seen,  and  the  descending  aorta;  have  united  to  form  a  single  dorsal 
vessel.  On  either  side  of  the  pharynx  are  seen  subdivisions  of  the  ccelom  which  will  form  the 
pleural  cavities.  These  cavities  are  separated  from  the  pericardial  cavity  by  the  septum  trans- 
versum  in  which  the  common  cardinal  veins  cross  to  the  sinus  venosus. 

The  folds  of  the  amnion  envelop  the  right  side  of  the  embryo  and  the  ectoderm  of  these 


CHICK   EMBRYO   OF   TWENTY-SEVEN    SEGMENTS 


71 


folds  now  forms  the  outer  layer  of  the  chorion  and  the  inner  layer  of  the  amnion.     The  meso- 
dermal folds  of  the  amnion  have  not  united. 


Chorion 


Spinal  cord 
Mes.  segment 


Common  . 
card-  vein 


Entoderm 
Myocardium 


Coeton 


Atrium 


Endothelium 
of  heart 


Fig.  59. — Transverse  section  through  the  sinus  venosus  and  common  cardinal  veins  of  a  fifty-hour 

chick  embryo.     X  50. 


Chorion 


Fig.  60. — Transverse  section  through  the  anlage  of  the  liver  of  a  thirty-six-hour  chick  embryo.     X  50. 


Section  through  the  Anlage  of  the  Liver  (Fig.  60).  —  In  this  section  the  cavity  of 
the  fore-gut  is  narrow,  the  gut  being  flattened  from  side  to  side.  Yentrad  there  are  evaginated 
from  the  entoderm  two  elongate  diverticula  which  form  the  anlages  of  the  liver.     On  either  side 


72 


THE    STUDY    OF    CHICK   EMBRYOS 


of  the  anlages  of  the  liver  are  sections  of  the  vitelline  veins  on  their  way  to  the  sinus  venosus 
at  a  higher  level  in  the  series.  Note  the  intimate  relation  between  the  entodermal  epithelium 
of  the  liver  and  the  endothelium  of  the  vitelline  veins.  In  later  stages,  as  the  liver  anlages 
branch,  there  is,  as  Minot  aptly  expresses  it,  "an  intercrescence  of  the  entodermal  cells  consti- 
tuting the  liver  and  of  the  vascular  endothelium"  of  the  vitelline  veins.  Thus  are  formed  the 
hepatic  sinusoids  of  the  portal  system,  which  surround  the  cords  of  hepatic  cells. 

The  septum  transversum  is  still  present  at  this  level  and  lateral  to  the  fore-gut  are  small 
body  cavities.     Lateral  to  the  body  cavities  appear  branches  of  the  posterior  cardinal  veins. 

Section  through  the  Cranial  Portion  of  the  Open  Intestine  (Fig.  61). — The  intestine 
is  now  open  ventrad,  its  splanchnopleure  passing  directly  over  to  that  of  the  vascular  area. 
The  folds  of  the  amnion  do  not  join,  leaving  the  amniotic  cavity  open.      The  dorsal  aorta 


Splanchnic  mesoderm 


Ectoderm 


Mes.segment 


Descending/ 
aorta 

Coelom 


Somato  phure 


Fig.  6i. — Transverse  section  through  the  cranial  portion  of  the  open  intestine  of  a  fifty-hour  chick 

embryo.     X  50. 


is  divided  by  a  septum  into  its  primitive  components,  the  right  and  left  aortcc.  The  ccelom  is  in 
communication  with  the  extra-embryonic  body  cavity. 

Section  through  the  Seventeenth  Pair  of  Mesodermal  Segments  (Fig.  62). — The 
body  of  the  embryo  is  now  no  longer  flexed  to  the  right.  On  the  left  side  of  the  figure  the 
mesodermal  segment  shows  a  dorso-lateral  muscle  plate.  The  median  and  ventral  portion  of 
the  segment  is  being  converted  into  mesenchyme.  On  the  left  side  appears  a  section  of  the 
primary  excretory  or  mesonephric  duct.  The  embryonic  somatoplcure  is  arched  and  will  form 
the  future  ventro-lateral  body  wall  of  the  embryo.  The  fold  lateral  to  the  arch  of  the  somato- 
pleure  gives  indication  of  the  later  approximation  of  the  ventral  body  walls,  by  which  the  embryo 
is  separated  from  the  underlying  layers  of  the  blastoderm. 

Section  through  the  Origin  of  the  Vitelline  Arteries  (Fig.  63). — At  this  level  the 
embryo  is  more  flattened  and  simpler  in  structure,  the  section  resembling  one  through  the 
mid-gut  region  of  a  thirty-six-hour  chick  (Fig.  47).  The  amniotic  folds  have  not  appeared. 
On  the  left  side  of  the  figure  the  vitelline  artery  leaves  the  aorta.     On  the  right  side  the  con- 


CHICK   EMBRYO    OF    TWENTY-SEVEN   SEGMENTS 


73 


nection  of  the  vitelline  artery  with  the  aorta  docs  not  show,  as  the  section  is  cut  somewhat 
obliquely.    The  other  structures  were  described  La  connection  with  Fig.  47. 

Section  Caudal  to  the  Mesodermal  Segments  (Fig.  64).  The  mesodermal  seg- 
ments are  replaced  by  the  segmental  zone,  a  somewhat  triangular  mass  of  undifferentiated 
mesoderm  from  which  later  are  formed  the  segments  and  nephrotomes.     The  notochord  is  larger, 


Afes.  Segment" 
Descending  aorta 

<5omatopleure 

Som.  mes 


Spinal  cord 

Ectoderm 
Notochord 


Somatic  mesoderm 


Splanchnopleure 
Coelo 


-Splanchnic  mesoderm 

^Entoderm 

Fig.  62. — Transverse  section  through  the  seventeenth  pair  of  mesodermal  segments  of  a  fifty-hour  chick. 

embryo.     X  50. 


Mes.segment. 
Nephrotome 


■Sp'wa I  cord 

Ectoderm 


Nephrotome 

Somatic  mesoderm 


Somatopleure. 


tJplanchnopleurc 

Aorta  Jj- Vitelline  artery 

Fig.  63. — Transverse  section  of  a  fifty-hour  chick  embryo  at  the  level  of  the  origin  of  the  vitelline 

arteries.     X  50. 


Descending  aorta 
Sim.  mesoderm 


Spinal  cord 

Ectoderm 


^Splanchnic  mesoderm 


Coelom 
Entoderm 


Notochord 

Fig.  64. — Transverse  section  of  a  fifty-hour  chick  embryo  through  the  last  pair  of  mesodermal  segments. 

X50. 


the  aortse  smaller,  and  a  few  sections  caudad  they  disappear.     Laterally  the  somatoplcurc  and 
splanchnopleurc  are  straight  and  separated  by  the  slit-like  ccelom. 

Section  through  the  Primitive  Node  Cranial  to  the  Hind-gut  (Fig.  65). — 'With 
the  exception  of  the  ectoderm,  the  structures  near  the  median  line  are  merged  into  an  undiffer- 
entiated mass  of  tissue.     The  cavity  of  the  neural  tube  and  its  dorsal  outline  may  still  be  seen, 


74 


THE    STUDY    OF    CHICK   EMBRYOS 


but  its  ventral  portion,  the  notochord,  mesoderm  and  entoderm,  blend  in  a  dense  mass  of  tissue 
which  is  characteristic  of  the  primitive  node.  Laterally  the  segmental  zone  and  the  various 
layers  are  differentiated. 

Section  Passing  through  the  Hind-gut  (Fig.  66). — In  this  embryo  the  caudal 
evagination  to  form  the  hind-gut  has  just  begun.  The  section  shows  the  small  cavity  of  the 
hind-gut  in  the  mid-line.  Its  wall  is  composed  of  columnar  entodermal  cells  and  it  is  an  out- 
growth of  the  entodermal  layer.  Dorsal  to  the  hind-gut  may  be  seen  undifferentiated  cells 
of  the  primitive  streak  continuous  dorsad  with  the  ectoderm,  ventrad  with  the  entoderm  of  the 
hind-gut  and  laterally  with  the  mesoderm. 

Neural  tube 

Bctoderm 

Segmental  stone 


pan    nop  u   ^oderm  Nohchordal  folate 

Fig.  65. — Transverse  section  of  a  fifty-hour  chick  embryo  through  the  primitive  node  cranial  to  the 

hind-gut.     X  50. 


In  the  chick  embryos  which  we  have  studied  there  are  large  areas  developed 
which  are  extra-embryonic,  that  is,  lie  outside  the  embryo.  The  splanchnopleure 
of  the  area  vasculosa,  for  instance,  forms  the  wall  of  the  yolk-sac,  incomplete  in 
the  early  stages.  The  amnion,  chorion  and  allantois  are  extra-embryonic  mem- 
branes which  make  their  appearance  at  the  fifty-hour  stage.  These  structures 
are  important  in  mammalian  and  human  embryos  and  a  description  of  their 
further  development  in  the  chick,  where  their  structure  and  mode  of  develop- 


Somcttic  mespnerm 


Primitive   node 

Ectoderm 


omatopleure 


Entoderm 


Splanchnopleure  ■ 

Hind -gut 
Fig.  66. — Transverse  section  passing  through  the  hind-gut  of  a  fifty-hour  chick  embryo.     X  S°- 


ment  is  primitive,  will  lead  up  to  the  study  of  mammalian  embryos  in  which  the 
amnion  and  chorion  are  precociously  developed. 

Amnion  and  Chorion. — These  two  membranes  are  developed  in  all  Amniote 
Vertebrates  (Reptiles,  Birds  and  Mammals).  They  are  derived  from  the  extra- 
embryonic somatopleure.  The  amnion  is  purely  a  protective  structure,  but  the 
chorion  of  mammals  has  a  trophic  function,  as  through  it  the  embryo  derives  its 
nourishment  from  the  uterine  wall.     Fig.  67  A  shows  the  amnion  and  chorion 


CHICK   EMBRYO    OF   TWENTY-SEVEN    SEGMENTS 


75 


developing.  The  head-fold  of  the  somatopleure  forms  first  and  envelops  the  head, 
the  tail-fold  makes  its  appearance  later.  The  two  folds  extend  laterad,  meet 
and  fuse  (Fig.  67  B).  The  inner  leaf  of  the  folds  forms  the  amnion,  the  remainder 
of  the  extra-embryonic  somatopleure  becomes  the  chorion.  The  actual  appear- 
ance of  these  structures  and  their  relation  to  the  embryo  we  have  seen  in  Figs.  60 
and  61.  The  amnion,  with  its  ectodermal  layer  inside,  completely  surrounds  the 
embryo  by  the  fourth  day,  enclosing  a  cavity  filled  with  amniotic  fluid  (Fig.  68). 
In  this  the  embryo  floats  and  is  thus  protected  from  injury.  The  chorion  is  of 
little  importance  to  the  chick.  It  is  at  first  incomplete  but  eventually  entirely 
surrounds  the  embryo  and  its  other  appendages. 

Yolk-sac  and  Yolk-stalk. — While  the  amnion  and  chorion  are  developing 
during  the    second    and    third 
day,  the  embryo  grows  rapidly.  A£ 

The  head-  and  tail-folds  elon- 
gate and  the  trunk  expands  lat- 
erally until  only  a  relatively  nar- 
row stalk  of  the  splanchno- 
pleure  connects  the  embryo  with 
the  yolk.  This  portion  of 
the  splanchnopleure  has  grown 
more  slowly  than  the  body  of 
the  embryo  and  is  termed  the 
yolk-stalk.  It  is  continuous 
with  the  splanchnopleure  which 
envelops  the  yolk  and  forms  the 
yolk-sac.  The  process  of  un- 
equal growth  by  which  the  em- 
bryo becomes  separated  from 
the  yolk  has  been  described  as  a 

process  of  constriction.  This,  as  Minot  points  out,  is  an  error.  The  splanchno- 
pleure at  first  forms  only  an  oval  plate  on  the  surface  of  the  yolk  but  eventually 
encloses  it.  In  Fig.  67,  C  and  D,  the  relation  of  the  embryo  to  the  yolk-sac  is 
seen  at  the  end  of  the  first  week  of  incubation.  The  vitelline  vessels  ramify  on 
the  surface  of  the  yolk-sac  and  through  them  all  the  food  material  of  the  yolk  is 
conveyed  to  the  chick  during  the  incubation  period  (about  twenty-one  days). 

Allantois. — We  have  seen  that  in  the  fifty-hour  chick  a  ventral  evagination, 
the  hind-gut,  develops  near  its  caudal  end  (Fig.  66).     From  it  develops  the  anlage 


Fig.  67. — Diagrams  showing  the  development  of  the 
amnion,  chorion  and  allantois  (Gegenbaur  in  McMurrich's 
''Human  Body")-  Af.,  amnion  folds;  A!.,  allantois;  A ;»., 
amniotic  cavitv;   Ds.,  volk-sac. 


76 


THE    STUDY   OF   CHICK  EMBRYOS 


of  the  allantois,  and,  as  it  is  an  outgrowth  of  the  splanchnopleure,  it  is  lined  with 
entoderm  and  covered  with  splanchnic  mesoderm  (Fig.  67).  It  develops  rapidly 
into  a  vesicle  connected  to  the  hind-gut  by  a  narrow  stalk,  the  allantoic  stalk. 
At  the  fifth  day  it  is  nearly  as  large  as  the  embryo  (Fig.  68).  Its  wall  flat- 
tens out  beneath  the  chorion  and  finally  it  lies  close  to  the  secondary  egg  mem- 
brane (shell)  but  is  attached  only  to  the  embryo.  The  functions  of  respiration 
and  excretion  are  ascribed  to  it.  In  its  wall  ramify  the  allantoic  vessels,  which 
have  been  compared  to  the  umbilical  arteries  and  veins  of  mammalian  embryos. 
The  chick  embryo  is  thus  protected  by  the  amnion  which  develops  from  the 


Fig.  68. — Diagram  of  a  chick  embryo  of  the  fifth  day  showing  amnion,  chorion  and  allantois  (Mar- 
shall). AN ,  inner  or  true  amnion;  A  V,  outer  margin  of  the  area  vasculosa;  AZ,  outer  or  false  amnion 
(chorion);  EM,  embryo;  SH,  shell  of  egg;  SM,  shell  membrane;  SV,  air  chamber;  TA,  allantois; 
YS,  yolk-sac. 


inner  leaf  of  the  folded  somatopleure  and  is  composed  of  an  inner  ectodermal  and 
an  outer  mesodermal  layer.  Nutriment  for  the  growth  of  the  embryo  is  supplied 
by  the  yolk-sac  and  carried  to  the  embryo  by  the  vitelline  veins.  The  allantois, 
which  takes  its  origin  from  the  splanchnopleure  of  the  hind-gut  and  is  composed 
of  an  inner  layer  of  entoderm  and  an  outer  layer  of  splanchnic  mesoderm,  func- 
tions as  an  organ  of  respiration  and  serves  as  a  reservoir  for  the  excreta  of  the 
embryonic  kidneys.  As  we  shall  see,  the  allantois  becomes  more  important,  the 
yolk-sac  less  important  in  some  mammals,  while  in  human  embryos  both  yolk  - 
sac  and  allantois  are  unimportant  when  compared  to  the  chorion. 


CHAPTER  IV 

THE  FETAL  MEMBRANES  AND  EARLY  HUMAN  EMBRYOS 

The  fetal  membranes  of  mammals  include  the  amnion,  chorion,  yolk-sac  and 
allantois,  structures  which  we  have  seen  are  present  in  chick  embryos.  Most 
important  in  mammals  is  the  manner  in  which  the  embryo  becomes  attached  to 
the  uterine  wall  of  the  mother  and  in  this  regard  mammalian  embryos  fall  into 
two  groups.  Among  the  Ungulates  or  hoofed  mammals  (example  the  pig)  the 
fetal  membranes  are  of  a  primitive  type,  resembling  those  of  the  chick.  Among 
Unguiculates  (clawed  animals  like  the  bat  and  rabbit)  and  Primates  (example 
Man)  the  fetal  membranes  of  the  embryo  show  marked  changes  in  development 
and  structure. 

FETAL  MEMBRANES  OF  THE  PIG  EMBRYO 
The  amnion  and  chorion  develop  very  much  as  in  the  chick  embryo  (Fig. 
67  A,  B).  A  fold  of  the  somatopleure  forms  very  early  about  the  whole  embryo. 
The  amnion  is  closed  in  embryos  with  but  a  few  pairs  of  segments,  but  for  some 
time  remains  attached  to  the  chorion  by  a  strand  of  tissue  (Keibel).  The  yolk- 
sac  develops  early  as  in  all  mammals.  In  the  pig  it  is  small  and  the  greater  part 
of  it  soon  degenerates.  It  is  important  only  in  the  early  growth  of  the  embryo, 
its  functions  then  being  transferred  to  the  allantois.  Branches  of  the  vitelline 
vessels  ramify  in  its  wall,  as  in  that  of  chick  embryos,  but  soon  degenerate. 
The  trunks  of  the  vitelline  vessels,  however,  persist  within  the  body  of  the  em- 
bryo. The  allantois,  developing  aS  in  the  chick  from  the  ventral  wall  of  the  hind- 
gut  (Fig.  67  A-D),  appears  when  the  embryo  is  still  flattened  out  on  the  germinal 
area.  In  an  embryo  3.5  mm.  long  it  is  crescent  shaped  and  as  large  as  the  em- 
bryo. It  soon  becomes  larger  and  its  convex  outer  surface  is  applied  to  the 
inner  surface  of  the  chorion.  As  these  surface  layers  are  composed  of  splanchnic 
mesoderm  they  fuse  more  or  less  completely.  A  pair  of  allantoic  veins  and  ar- 
teries branch  in  the  splanchnic  layer  of  the  allantois.  These  branches  are  brought 
into  contact  with  and  invade  the  mesodermal  layer  of  the  chorion.  The  outer 
ectodermal  layer  of  the  chorion  in  the  meantime  has  closely  applied  itself  to  the 
uterine  epithelium,  the  ends  of  the  uterine  cells  fitting  into  depressions  in  the 

77 


78 


THE   FETAL   MEMBRANES   AND   EARLY  HUMAN   EMBRYOS 


chorionic  cells  (Fig.  69) .    When  the  allantoic  circulation  is  established,  waste  prod- 
ucts given  off  from  the  blood  of  the  embryo  must  pass  through  the  epithelia  of 


Spinal  cord 


Mes.segment 

Amniotic  cavity 

Upper 
limb  bud 


FbjtamJinaJ 
vein 

Dorsal  ClorTa 


Glomerulus 
Rumbilical  Vein 


Hind-gut 


Somatic  and 
Splanchnic 
mesoderm 


YolK-sac 


Entoderm 


-Chorionic  mesoderm 
Chorionic  ectoderm 
Uterine  1  epithelium 
■  Tunica  propria  of  uterus 

Fig.  69.— A,  Transverse  section  through  the  yolk-sac  and  stalk  of  a  5  mm.  pig  embryo  showing 
attachment  of  amnion.  B,  Diagram  of  the  fetal  membranes  and  allantoic  placenta  of  a  pig  embryo  in 
median  sagittal  section  (based  on  figures  of  Heisler  and  Minot). 


UMBILICAL   CORD 


79 


both  chorion  and  uterus  to  be  taken  up  by  the  blood  of  the  mother.  In  the  same 
way  nutritive  substances  and  oxygen  must  pass  from  the  maternal  blood  through 
these  layers  to  enter  the  allantoic  vessels.  This  exchange  does  take  place,  how- 
ever, and  thus  in  Ungulates  the  allantois  has  become  important  not  only  as  an 
organ  of  respiration  and  excretion  but  as  an  organ  of  nutrition.  Through  its 
vessels  it  has  taken  on  a  function  belonging  to  the  yolk-sac  in  birds,  and  we  now 
see  why  the  yolk-sac  becomes  a  rudimentary  structure  in  the  higher  mammals. 
Excreta  from  the  embryonic  kidneys  are  passed  into  the  cavity  of  the  allantois 
which  is  relatively  large.  The  name  is  derived  from  a  Greek  word  meaning  saus- 
age-like, from  its  form  in  some  animals.  The  chorion  is  important  only  as  it 
brings  the  allantois  into  close  relation  to  the  uterine  wall,  but  in  man  we  shall 
see  that  it  plays  a  more  important  role. 


UMBILICAL  CORD 

In  their  early  development  the  relation  of  the  amnion,  allantois  and  yolk- 
sac  to  each  other  and  to  the  embryo  is  much  the  same  as  in  the  chick  of  five 
days  (Fig.  68).  With  the  increase  in  size  of  the  embryo,  however,  the  somato- 
pleure  in  the  region  of  the  attachment  of  the  amnion  grows  ventrad.  As  a 
result,  it  is  carried  downward  with  the  ccelom  about  the  yolk-sac  and  allantois, 
forming  the  umbilical  cord.  Thus  in  a  pig  embryo  10  to  12  mm.  long  the  amnion 
is  attached  at  a  circular  line  about  these  structures  some  distance  from  the  body 
of  the  embryo.  The  ccelom  at  first  extends  ventrad  into  the  cord,  but  later  the 
mesodermal  layers  of  amnion,  yolk-stalk  and  allantois  fuse  and  form  a  solid  cord 
of  tissue.  This  is  the  umbilical  cord  of  fetal  life  and  its  point  of  attachment  to  the 
body  is  the  umbilicus  or  navel.  The  cord  is  covered  by  a  layer  of  ectoderm  con- 
tinuous with  that  of  the  amnion  and  of  the  embryo  and  contains,  embedded  in  a 
mesenchymal  (mucous)  tissue  (1)  the  yolk-stalk  and  (in  early  stages)  its  vitelline 
vessels;  (2)  the  allantoic  stalk;  (3)  the  allantoic  vessels.  These,  two  arteries 
and  a  single  large  vein,  are  termed  from  their  position  the  umbilical  vessels.  At 
certain  stages  (Figs.  117  and  118)  the  gut  normally  extends  into  the  ccelom  of 
the  cord,  forming  an  umbilical  hernia.  Later,  it  returns  to  the  ccelom  of  the 
embryo  and  the  cavity  of  the  cord  disappears.  The  umbilical  cord  of  the  pig 
is  very  short. 

Human  Umbilical  Cord. — This  develops  like  that  of  the  pig  and  may  attain  a 
length  of  more  than  50  cm.  It  becomes  spirally  twisted,  just  how  is  not  kno%vn. 
In  embryos  from  10  mm.  to  40  mm.  long  the  gut  extends  into  the  ccelom  of  the 


80  THE   FETAL   MEMBRANES   AND   EARLY  HUMAN  EMBRYOS 

cord  (Fig.  172).  At  the  42  mm.  stage,  according  to  Lewis  and  Mall,  the  gut  re- 
turns to  the  ccelom  of  the  body.  The  mucous  tissue  peculiar  to  the  cord  arises 
from  mesenchyme.  It  contains  no  capillaries  and  no  nerves,  but  embedded  in  it 
are  the  large  umbilical  vein,  the  two  arteries,  the  allantois  and  the  yolk-stalk. 
The  umbilical  cord  may  become  wound  about  the  neck  of  the  fetus,  causing  its 
death  and  abortion,  or  by  coiling  about  the  extremities  it  may  lead  to  their  atrophy 
or  amputation. 


EARLY  HUMAN  EMBRYOS  AND  THEIR  MEMBRANES 
Referring  to  the  blastodermic  vesicle  of  the  mammal  (Figs.  16  and  17),  we 
find  it  consists  of  an  outer  layer,  which  we  have  called  the  trophectoderm,  and  the 
inner  cell  mass.  The  trophectoderm  forms  the  primitive  ectodermal  layer  of  the 
chorion  in  the  higher  mammals  and  probably  in  man.  From  the  inner  cell  mass 
are  derived  the  primary  ectoderm,  entoderm  and  mesoderm.  In  the  earliest 
known  human  embryos  described  by  Teacher,  Bryce,  and  Peters,  the  germ  layers 
and  amnion  are  present,  indicating  that  they  are  formed  very  early.  We  can 
only  guess  at  their  early  origin  by  what  we  know  from  other  mammals.  The 
diagrams  (Fig.  70  A  and  B)  show  two  hypothetical  stages  seen  in  median  longi- 
tudinal section.  In  the  first  stage  (A)  the  blastodermic  vesicle  is  surrounded  by 
the  trophectoderm  layer.  The  inner  cell  mass  is  differentiated  into  a  dorsal  mass 
of  ectoderm  and  a  ventral  mass  of  entoderm.  Mesoderm  more  or  less  completely 
fills  the  space  between  entoderm  and  trophoderm.  It  is  assumed  that  as  the 
embryo  grows  (Fig.  70  B)  a  split  occurs  in  the  mass  of  ectoderm  cells,  giving  rise 
to  the  amniotic  cavity  and  dividing  these  cells  into  the  ectodermal  layer  of  the 
embryo  and  into  the  extra-embryonic  ectoderm  of  the  amnion.  At  the  same  time, 
a  cavity  may  be  assumed  to  form  in  the  entoderm,  giving  rise  to  the  primitive  gut. 
About  this  stage  the  embryo  embeds  itself  in  the  uterine  mucosa.  In  the  third 
stage,  based  on  Peter's  embryo  (Fig.  70  C),  the  extra-embryonic  mesoderm  has 
extended  between  the  trophectoderm  and  the  ectoderm  of  the  amnion  and  the 
extra-embryonic  ccelom  appears.  The  amniotic  cavity  has  increased  in  size  and 
the  embryo  is  attached  to  the  trophectoderm  by  the  unsplit  layer  of  mesoderm 
between  the  ectoderm  of  the  amnion  and  the  trophectoderm  of  the  chorion.  The 
latter  shows  thickenings  which  are  the  anlages  of  the  chorionic  villi  surrounded 
by  trophoderm  cells.  In  the  fourth  stage,  based  on  Graf  Spee's  embryo  (D), 
the  chorionic  villi  are  longer  and  branched.  The  mesoderm  now  remains  unsplit 
only  at  the  posterior  end  of  the  embryo,  where  it  forms  the  body-stalk  peculiar 


EARLY   HUMAN   EMBRYOS   AND   THEIR   MEMBRANES 


8l 


to  Unguiculates  and  Primates.  It  connects  the  mesoderm  of  the  embryo  with 
the  mesoderm  of  the  chorion.  Into  it  there  has  grown  from  the  gut  of  the  em- 
bryo the  entodermal  diverticulum  of  the  allantois. 

The  Chorion. — The  human  chorion  is  derived  directly  from  the  outer  troph- 
ectoderm  layer  of  the  blastodermic  vesicle  and  from  the  extra-embryonic  somatic 
mesoderm.     Its  early  structure  resembles  that  of  the  pig's  chorion.     The  troph- 


Ectode 


Amniotic  cavity 
Coelom 
Trophecto  derm 
Archenteron 


Entoderm 
Mesoderm 


D 


Ectoderm  of  amnion 
Ectoderm  of  embryo 


Amniotic  cavity 


Troph  ectoderm 


YolK-Sac 
Entoderm 


Splanchnic 
mesoderm 


Ectoderm  of  embryo 
Cavity  of  amnion 
Mesoderm  of  amnion 

Ectoderm  ofchonon 

Cavity  of 'yolK 
sac 

Entoderm  of 
yol  K-jac 

Mesoderm  of 


Allantoi  s 


BodystalK 


S     Extraembryonic     yolKSac 
a —    rnelam  ' 


Extraembryonic 
coelom 

Chorionic 

mesoderm        Mesoderm  of  Ck 
Trophoderm 

Chor 


Fig.  70. — Four  diagrams  showing  hypothetical  stages  of  early  human  embryos  (based  on  figures  of 

Robinson  and  Minot). 


ectoderm  of  the  human  embryo  early  gives  rise  to  a  thickened  outer  layer,  the 
trophoderm  (syncytial  and  nutrient  layer) .  When  the  developing  embryo  comes 
into  contact  with  the  uterine  wall  the  trophoderm  destroys  the  maternal  tissues. 
The  destruction  of  the  uterine  mucosa  serves  two  purposes:  (1)  the  embedding 
and  attachment  of  the  embryo,  it  being  grafted,  so  to  speak,  to  the  uterine  wall; 
and  (2)  it  supplies  the  embryo  with  a  new  source  of  nutrition.  To  obtain  nutri- 
6 


82 


THE   FETAL   MEMBRANES   AND   EARLY   HUMAN   EMBRYOS 


ment  to  better  advantage,  there  grow  out  from  the  chorion  into  the  uterine 
mucosa  branched  processes  or  villi.  The  villi  are  bathed  in  maternal  blood,  and 
in  them  blood-vessels  are  developed,  the  trunks  of  which  pass  to  and  from  the 
embryo  as  the  umbilical  vessels.     The  embryo  receives  its  nutriment  and  oxygen, 


Inner eel '/-mass 


Entode 


Trophoblast 


Inner  eell-mass      Trophoblast 


Embryonic  ectoderm    Entoderm 


Maternal  bloodvessels 


Syncytiotrophoblast 

Cijtotrophoblast 


Embryonic  ectoderm  Entoderm 

Fig.  71. — Section  showing  three  stages  in  the  formation  of  the  amnion  of  bat  embryo  (after  Van  Beneden). 


and  gets  rid  of  waste  products  through  the  walls  of  the  villi.  The  region  where 
the  attachment  of  the  chorionic  villi  to  the  uterine  wall  persists  during  fetal  life 
is  known  as  the  placenta.  It  will  be  described  later  with  the  decidual  membranes 
of  the  uterus.     We  saw  how  the  allantois  of  Ungulates  had  assumed  the  nutritive 


EARLY   HUMAN   EMBRYOS   AND    THEIR   MEMBRANES 


83 


functions  performed  by  the  yolk-sac  in  birds,  with  a  consequent  degeneration  of 
the  ungulate  yolk-sac.  In  man  and  Unguiculates,  the  functions  of  the  allantois 
are  transferred  to  the  chorion  and  the  allantois  in  turn  becomes  a  rudimentary 
structure. 

The  Amnion. — This  is  formed  precociously  in  Unguiculates  and  in  a  manner 
quite  different  from  its  mode  of  origin  in  Ungulates  and  birds.  It  is  assumed  that 
its  cavity  arises  as  a  split  in  the  primitive  ectoderm  of  human  embryos,  as  in  bat 
embryos  (Fig.  71).  Later,  a  somatic  layer  of  mesoderm  envelops  its  ectodermal 
layer,  its  component  parts  then  being  the  same  as  in  birds  and  Ungulates,  an 
inner  layer  of  ectoderm  and  an  outer  layer  of  mesoderm  (Fig.  70  D).     It  becomes 


—        —         «9  c»>     f>     J    °- 


Fig.  72. — Section  of  embryonic  rudiment  in  Peters'  ovum  (first  week)  (after  Peters),  cct,  ectoderm 
of  chorion;  mes,  mesoderm;  am,  amnion;  cm.  pi.,  embryonic  plate;  y.s..  yolk-sac;  cut.  entoderm; 
ex.  cce.,  portion  of  extra-embryonic  ccelom  limited  by  a  strand  of  the  magma  reticulare. 

a  thin,  pellucid,  non- vascular  membrane  and  about  a  month  before  birth  is  in 
contact  with  the  chorion.  It  then  contains  from  one-half  to  three-fourths  of  a 
liter  of  amniotic  fluid,  the  origin  of  which  is  unknown.  During  the  early  months 
of  pregnancy  the  embryo,  suspended  by  the  umbilical  cord,  floats  in  the  amniotic 
fluid.  The  embryo  is  protected  from  maceration  by  a  white  fatty  secretion,  the 
vernix  caseosa. 


At  birth  the  amnion  is  ruptured  either  normally  or  artificially.  If  not  ruptured, 
the  child  may  be  born  enveloped  in  the  amnion  popularly  known  as  a  veil  or  ''caul." 
The  amniotic  fluid  may  be  present  in  excessive  amount,  the  condition  being  known  as 
hydr amnios.  If  less  than  the  normal  amount  of  fluid  is  present,  the  amnion  may  adhere 
to  the  embryo  and  produce  malformations.  It  has  been  found,  too,  that  fibrous  bands  or 
cords  of  tissue  may  extend  across  the  amniotic  cavity  and,  pressing  upon  parts  of  the  embryo 


84 


THE   FETAL  MEMBRANES   AND   EARLY  HUMAN   EMBRYOS 


during  its  growth,  may  cause  scars  and  splitting  of  eyelids  or  lips.  Such  amniotic  threads 
may  even  amputate  a  limb  or  cause  the  bifurcation  of  a  digit  producing  a  type  of  polydac- 
tylism. 

The  Allantois.— The  allantois  appears  very  early  in  the  human  embryo  be- 


Yolk-sac 


Neural  groove   — j — r~r 


Neurenteric  canal 
Primitive  streak 
Body-stalk 


r^u. 


$&y- 


--! 


Villi  of  chorion 


Chorion 


t^-  Mesoderm 


Body-stalk 

Primitive 

streak 


Allantois 
Yolk-sac 


Mesoderm 


Fig.  73.— Views  of  a  human  embryo  1.54  mm.  long.     A,  dorsal  surface;    B,  median  sagittal  section 

(Graf  Spee). 


EARLY   HUMAN    EMBRYOS    AND    THEIR    MEMBRANES 


85 


fore  the  development  of  the  fore-gut  or  hind-gut.  In  Peter's  embryo  the  amnion, 
chorion  and  yolk-sac  are  present  but  not  the  allantois  (Fig.  72).  In  an  embryo 
1.54  mm.  long,  described  by  Von  Spee  (Fig.  73  A,  B),  there  is  no  hind-gut,  but  the 
allantoic  diverticulum  of  the  entoderm  has  invaded  the  mesoderm  of  the  body- 
stalk.  This  embryo,  seen  from  the  dorsal  side  with  the  amnion  cut  away,  shows 
a  marked  neural  groove  and  primitive  streak.  In  front  of  the  primitive  knot  a 
pore  is  figured  leading  from  the  neural  groove  into  the  primitive  intestinal 
cavity,  hence  called  the  neurenteric  canal.  The  fore-gut  and  head-fold  have 
formed  at  this  stage  and  there 
are  branched  chorionic  villi. 

A  reconstruction  by  Dandy 
of  Mali's  embryo,  about  2  mm. 
long  with  seven  pairs  of  seg- 
ments, shows  well  the  embryonic 
appendages  (Fig.  74).  The  fore- 
and  hind-gut  are  well  developed, 
the  amniotic  cavity  is  large,  and 
the  yolk-sac  still  communicates 
with  the  gut  through  a  wide  open- 
ing. The  allantois  is  present  as  a 
long  curved  tube  somewhat  di- 
lated near  its  blind  end  and  em- 
bedded in  the  mesoderm  of  the 
body-stalk.  As  the  hind-gut  de- 
velops, the  allantois  comes  to 
open  into  its  ventral  wall.  A 
large  umbilical  artery  and  vein 
are  present  in  the  body-stalk. 

In  an  embryo  of  23  somites 
2.5  mm.  long,  described  by  Thompson,  the  allantois  has  elongated  and  shows 
three  irregular  dilatations  (Fig.  75).  A  large  cavity  never  appears  distally  in 
the  human  allantois  as  in  Ungulates.  When  it  becomes  included  in  the  umbilical 
cord  its  distal  portion  is  tubular  and  it  eventually  atrophies.  That  part  of  the 
allantois  extending  from  the  umbilicus  to  the  cloaca  of  the  hind-gut  takes  part 
in  forming  the  urogenital  sinus,  the  bladder  and  the  urachus,  a  rudiment  extend- 
ing as  a  solid  cord  from  the  fundus  of  the  bladder  to  the  umbilicus.  According 
to  Felix,  the  allantois  forms  only  the  urachus  and  a  portion  of  the  bladder. 


Fig.  74. — A  human  embryo  of  2  mm.  in  median 
sagittal  section  (adapted  from  reconstructions  of  Mall's 
embryo  by  F.  T.  Lewis  and  Dandy). 


86 


THE    FETAL   MEMBRANES   AND   EARLY  HUMAN   EMBRYOS 


The  human  allantois  is  thus  small  and  rudimentary  as  compared  with  that 
of  birds  and  Ungulates.  As  we  have  seen,  the  cavity  is  very  large  in  the  pig,  and 
Haller  found  an  allantoic  sac  two  feet  long  connected  with  a  goat  embryo  of  two 
inches.     In  human  embryos  it  appears  very  early  and  is  not  free  but  embedded 

in  the  body-stalk.  Its  functions,  so 
important  in  birds  and  Ungulates,  are 
in  man  performed  by  the  chorion. 


Neural  folds 


Neurenteric  canal 


Fig.  75. — Median  sagittal  section  of  a  2.5 
mm.  human  embryo  showing  digestive  tract 
(after  Thompson).  X  40.  All.,  allantois; 
CI.,  cloaca;  C.  per.,  pericardial  cavity;  Div.hep., 
hepatic  diverticulum;  D.  v.,  ductus  vitellinus 
(yolk-stalk);  gl.  th.,  thyreoid  gland;  Men.  cl., 
cloacal  membrane;  Ph.,  pharynx;  Sept.  lr.,  sep- 
tum transversum. 


Fig.  76. — Human  embryo  of  2.1 1  mm.  (Eternod). 


Yolk-Sac  and  Yolk-Stalk.— In  the 

youngest  human  embryos  described 
(Peters)  the  entoderm  forms  a  some- 
what elongated  vesicle.  With  the 
development  of  the  fore-gut  and  hind- 
gut  in  embryos  of  1.54  and  2  mm.  (Figs.  73  and  74),  the  entodermal 
vesicle  is  divided  into  the  dorsal  intestine  and  ventral  yolk-sac,  the  two 
being  connected  by  a  somewhat  narrower  region.  This  condition  persists  in 
an  embryo  of  2.5  mm.  long  (Fig.  75).  In  the  figure  most  of  the  yolk-sac 
has  been  cut  away.  An  embryo  with  9  pairs  of  segments,  with  three  brain 
vesicles  and  with  the  amnion  cut  away  is  seen  in  Fig.  76.  The  relation 
of  the  fetal  appendages  to  the  embryo  shows  well  in  the  embryo  of  Coste  (Fig. 


EARLY  HUMAN   EMBRYOS   AND    THEIR   MEMBRA  \  I  - 


87 


77).  The  dorsal  convexity  is  probably  abnormal.  A  robust  body-stalk  attaches 
the  embryo  to  the  inner  wall  of  the  chorion.  With  the  growth  of  the  head-  and 
tail-folds  of  the  embryo,  there  is 
an  apparent  constriction  of  the 
yolk-sac  where  it  joins  the  em- 
bryo. This  will  become  more 
marked  in  later  stages  and  form 
the  yolk-stalk.  His's  embryo, 
2.6  mm.  long,  shows  the  relative 
size  of  yolk-sac  and  embryo  and 
the  yolk-stalk  (Fig.  78).  The 
relations  of  the  fetal  membranes 
to  the  embryo  are  much  the  same 
as  in  the  chick  embryo  of  five 
days,  save  that  the  allantois  of 
the  human  embryo  is  embedded 
in  the  body-stalk.  The  embryo 
shows    a   regular    convex   dorsal 

curvature,  there  is  a  marked  cephalic  bend  in  the  region  of  the  mid-brain  and 
there  are  three  gill  clefts.     The  head  is  twisted  to  the  right,  the  tail  to  the 


Fig.  77. — Human  embryo  at  the  commencement 
of  the  third  week  (from  His,  after  Coste).  X  15.  A, 
inner  or  true  amnion;  A.s.,  body-stalk;  //,  heart;  V, 
blood-vessel  on  yolk-sac;    Y.s.,  yolk-sac. 


Amnion 


Branchial  clefts  1-3 


Body-stalk 


Maxillary  process 

Mandibular  process 
Heart 


Yolk-sac 

Fig.  78. — Human  embryo  2.6  mm.  long  showing  amnion,  yolk-stalk  and  body-stalk  (His). 


88 


THE   FETAL   MEMBRANES   AND   EARLY  HUMAN  EMBRYOS 


left.  At  the  side  of  the  oral  sinus  are  two  large  processes;  the  dorsal  of  these 
is  the  maxillary,  the  ventral  the  mandibular  process.  The  heart  is  large  and 
flexed  in  much  the  same  way  as  the  heart  of  the  fifty-hour  chick  embryo. 


Fig.   70. — Yolk-sac  and-stalk  of  a   20  mm.  human  embryo.     X  11. 


Mid-brain 


In  later  stages,  with  the  development  of  the  umbilical  cord,  the  yolk-stalk 
becomes  a  slender  thread  extending  from  the  dividing  line  between  the  fore-  and 
hind-guts  to  the  yolk-sac  or  umbilical  vesicle  (Fig.  114).  It  loses  its  attach- 
ment to  the  gut  in  7  mm.  embryos.     A  blind  pocket  may  persist  at  its  point  of 

union  with  the  intestine  and  is  known 
as  Meckel's  diverticulum,  a  struc- 
ture of  clinical  importance  because 
it  may  telescope  and  cause  the  oc- 
clusion of  the  intestinal  lumen.  The 
yolk-stalk  may  remain  embedded  in 
the  umbilical  cord  and  extend  some 
distance  to  the  yolk-sac  which  is 
found  between  the  amnion  and 
chorion  (Fig.  79).  The  yolk-sac 
may  be  persistent  at  birth. 


Fore-brain  ~~i 


Stomodaum 

Mandibular 
process 

I/cart 


Hind-brain 

uditory  vesicle 


Branchial 
arches 


■Amnion  (cut) 


Body-stalk 


THE  ANATOMY  OF  A  4.2  MM.  HUMAN 
EMBRYO 


Fig.  80. — Left  side  of  a  human  embryo  of  4.2  mm 
(His). 


This  embryo,   studied  and  de- 
scribed   by   His,    is    probably   not 
quite  normal.     It  shows  a  concave 
dorsal  flexure  which   Keibel  regards  as  due  to  distortion.     Viewed   from   the 
left  side  (Fig.  80),  with  the  amnion  cut  away  close  to  its  line  of  attachment, 
there  may  be  seen  the  yolk-stalk,  and  a  portion  of   the  yolk-sac  and  of  the 


THE    ANATOMY    OF    A    4.2    MM.    IH'MAN    EMBRYO  89 

body-stalk.  There  is  an  indication  of  the  primitive  segments  along  the  dorso- 
lateral line  of  the  trunk.  The  head  is  bent  ventrad  almost  at  right  angles 
in  the  mid-brain  region  (cephalic  flexure).  There  are  also  marked  cervical  and 
caudal  llexures,  the  trunk  ending  in  a  short  blunt  tail.  The  heart  is  large  and 
flexed  as  in  the  earlier  stage.  Three  gill  clefts  separate  the  four  branchial  arches. 
The  first  has  developed  two  ventral  processes.  Of  these  the  maxillary  process 
is  small  and  may  be  seen  dorsal  to  the  stomodaeum.  The  mandibular  process  is 
large  and  has  met  its  fellow  of  the  right  side  to  form  the  mandible  or  lower  jaw. 
Dorsal  to  the  second  gill  cleft  may  be  seen  the  position  of  the  oval  otocysl,  now 
a  closed  sac.  Opposite  the  atrial  portion  of  the  heart  and  in  the  region  of  the 
caudal  flexure  bud-like  outgrowths  indicate  the  anlages  of  the  upper  and  lower 
extremities. 

Central  Nervous  System  and  Sense  Organs. — The  neural  tube  is  closed 
throughout  its  extent  and  is  differentiated  into  brain  and  spinal  cord.  The 
brain  tube  or  encephalon  is  divided  by  constrictions  into  four  regions  or  vesicles 
as  in  the  fifty-hour  chick  (Fig.  55).  Of  these,  the  most  cephalad  is  the  telenceph- 
alon. It  is  a  paired  outgrowth  from  the  fore-brain,  the  persisting  portion  of 
which  is  the  dicnccplialon.  The  mid-brain  or  mesencephalon  located  at  the 
cephalic  flexure  is  not  subdivided.  The  hind-brain,  or  rhombencephalon, 
which  is  long  and  continuous  with  the  spinal  cord,  later  is  subdivided  into  the 
metencephalon  (region  of  the  cerebellum  and  pons)  and  myelenccphalon  (medulla 
oblongata).  The  spinal  cord  forms  a  closed  tube  extending  from  the  brain  to 
the  tail  and  containing  the  neural  cavity,  flattened  from  side  to  side. 

The  eye  is  represented  by  the  optic  vesicles  and  the  thickened  ectodermal 
anlage  of  the  lens.  Its  stage  of  development  is  between  that  of  the  thirty->ix 
and  fifty-hour  chick  embryos. 

The  otocyst  is  a  closed  sac,  no  longer  connected  with  the  outer  ectoderm  as 
in  the  fifty-hour  chick. 

Digestive  Canal. — In  a  reconstruction  of  the  viscera  viewed  from  the  right 
side  (Fig.  81),  the  entire  extent  of  the  digestive  canal  may  be  seen.  The  pharyn- 
geal membrane  which  we  saw  developed  in  the  chick  between  the  stomodaeum 
and  the  pharynx  has  broken  through  so  that  these  cavities  are  now  in  communi- 
cation. The  fore-gut.  which  extends  from  the  oral  cavity  to  the  yolk-stalk  is 
differentiated  into  pharynx,  trachea  and  lungs,  esophagus  and  stomach,  small 
intestine  and  digestive  glands  (pancreas  and  liver).  The  gut  is  suspended  from 
the  dorsal  body  wall  by  the  dorsal  mesentery. 

The  ectodermal  limits  of  the  oral  cavitv  are  indicated  dorsad  bv  the  diverti- 


9° 


THE    FETAL   MEMBRANES    AND    EARLY   HUMAN   EMBRYOS 


culum  of  the  hypophysis  (Rathke's  pocket).  The  fore-gut  proper  begins  with 
a  shallow  out-pocketing  known  as  Seessel's  pocket.  As  the  pharyngeal  mem- 
brane disappears  between  these  two  pockets,  it  would  seem  that  Seessel's  pocket 
represents  the  persistence  of  the  blind  anterior  end  of  the  fore-gut.  No  other 
significance  has  been  assigned  to  it. 


MetencephaJo, 


/lortic  arches 
/.Z.3.4.6 


A/ofochord 


Hind-gut 


/Mesencephalon  % cephalic  flexure 
Hypophysis 

Diencephalon 

Int.  Carotid  artery 

Optic  vesicle 

Trosen  cephalon 
Mouth  cavity 

Pharyngeal 
pouches  l-*r 

Ventral  aorta- 

Atrium    of 
heart 

Umbilical 
Vein 

Liver  an/age 

•Jplanchmc 
mesoderm 

Mid-gut 
Entoderm  of 

yolk-stalk 

Tail  aut 
Umbilical  artery 
Meson ephric  duct 


Joaca. 

Allantois 

Fie.  81. — Diagrammatic  reconstruction  of  a  4.2  mm.  human  embryo,  viewed  from  the  right  side  (adapted 

from  a  model  by  His). 

The  pharynx  is  widened  laterally  and  at  this  stage  shows  four  pharyngeal 
pouches.  Later  a  fifth  pair  of  pouches  is  developed  (Fig.  82).  The  four  pairs 
of  pharyngeal  pouches  are  important  as  they  form  respectively  the  following 
adult  structures:  (1)  the  Eustachian  tubes;  (2)  the  palatine  tonsils;  (3)  the 
thymus  anlages;  (4)  the  parathyreoids  or  epithelial  bodies.  Between  the  pharyn- 
geal pouches  are  the  five  branchial  arches  in  which  are  developed  five  pairs  of 
aortic  arches.     Between  the  bases  of  the  first  and  second  branchial  arches,  on 


THE  ANATOMY  OF  A  4.2  MM.  HUMAN  EMBRYO 


91 


the  floor  of  the  pharynx,  is  developed  the  tuberculum  impar  which  may  form  a 
portion  of  the  anterior  part  of  the  tongue.  Posterior  to  this  unpaired  anlage  of 
the  tongue  there  grows  out  ventrally  the  anlage  of  the  thyreoid  gland.  From 
the  caudal  end  of  the  trachea  have  appeared  ventrally  the  lung  buds.  The 
trachea  is  still  largely  a  groove  in  the  ventral  wall  of  the  pharynx  and  esophagus. 
Caudal  to  the  lungs  a  slight  dilation  of  the  digestive  tube  indicates  the  position 


Mouth  cavity 


Pharyngeal 
pouches  /-4- 


Trachea 


Luna    bud 


Hepatic  diverticulum 
Ventral  pancreas 

Mesonephric  tubule 
with  glomerulus 

Hind-gut 
Allantois 


Tail- gut 


Thyreoid  anlage 


Esophagus 

Stomach 

Dorsal  pancreas 
YolK-stalK 

Mesonephros 
Mesonephric  duct 


a 


oaca. 


FIG.  82. — Diagrammatic  ventral  view  of  pharynx,  digestive  tube  and  mesonephroi  of  a  4-5  mm. 
embryo  (based  on  reconstructions  by  Grosser  and  His).  The  liver  and  yolk-sac  are  cut  away.  The 
tubules  of  the  right  mesonephros  are  shown  diagrammatically. 

of  the  stomach.  The  liver  diverticulum  has  grown  out  from  the  fore-gut  into 
the  ventral  mesentery  cranial  to  the  wall  of  the  yolk-stalk.  It  is  much  larger 
than  in  the  fifty-hour  chick,  where  we  saw  its  paired  anlage  cranial  to  the 
fovea  cardiaca,  and  is  separated  from  the  heart  by  the  septum  transvcrsum.  The 
small  intestine  between  the  liver  and  yolk-stalk  is  short  and  broad.  In  later 
stages  it  becomes  enormously  elongated  as  compared  with  the  rest  of  the  diges- 
tive tube.     The  yolk-stalk  is  still  broad  and  wide.     The  region  of  its  attachment 


92  THE   FETAL   MEMBRANES   AND   EARLY  HUMAN  EMBRYOS 

to  the  gut  corresponds  to  the  open  mid-gut  of  the  chick  embryo.  The  hind-gut  and 
tail-fold  of  this  embryo  are  greatly  elongated  as  compared  with  the  chick  embryo 
of  fifty  hours.  The  hind-gut  terminates  blindly  in  the  tail.  Near  its  caudal 
end  it  is  dilated  to  form  the  cloaca.  Into  the  ventral  side  of  the  cloaca  opens  the 
stalk  of  the  allantoic.  Dorso-laterally  the  primary  excretory  (Wolffian)  ducts 
which  we  saw  developed  in  the  fifty-hour  chick  have  connected  with  and  open 
into  the  cloaca.  Caudal  to  the  cloaca  on  the  ventral  side  is  the  cloacal  mem- 
brane, which  later  divides  and  breaks  through  to  form  the  genital  aperture  and 
anus.  That  part  of  the  hind-gut  between  the  cloaca  and  the  yolk-stalk  forms 
the  rectum,  colon,  caecum,  and  appendix,  with  a  portion  of  the  small  intestine 
(ileum) . 

Urogenital  Organs.— We  have  seen  that  the  primary  excretory  (Wolffian) 
ducts  open  into  the  cloaca.  These  are  the  ducts  of  the  mid-kidney  or  meso- 
nephros.  At  this  stage  the  nephrotomes,  which  in  the  chick  embryos  formed  the 
anlages  of  these  ducts,  are  also  forming  the  kidney  tubules  of  the  mesonephros 
which  open  into  the  ducts  (Fig.  82).  The  mid-kidneys  project  into  the  peri- 
toneal cavity  as  ridges  on  each  side.  A  thickening  of  the  mesothelium  along  the 
median  halves  of  the  mesonephroi  forms  the  anlage  of  the  genital  glands  or 
gonads  (Fig.  213). 

Circulatory  System. — The  heart  is  an  S-shaped  double  tube  as  in  the  fifty- 
hour  chick.  The  outer  myocardium  is  confined  to  the  heart,  while  the  inner 
endothelial  layer  is  continuous,  at  one  end  with  the  veins,  at  the  other  end  with 
the  arteries.  The  disposition  of  the  heart  tube  is  well  seen  in  a  ventral  view  of  a 
younger  embryo  (Fig.  83).  The  veins  enter  the  sinus  venosus  just  cranial  to 
the  yolk-sac.  Next  in  front  is  the  atrium  with  the  convexity  of  its  flexure  di- 
rected cephalad.  The  ventricular  portion  of  the  heart  is  U-shaped  and  is  flexed 
to  the  right  of  the  embryo.  To  the  left  is  the  ventricular  limb,  to  the  right  is 
the  bulbus.  The  arteries  begin  with  the  ventral  aorta  which  bends  back  to  the 
midline  and  divides  into  five  branches  on  each  side  of  the  pharynx  (Figs.  83  and 
84) .  These  are  the  aortic  arches  and  they  unite  dorsally  to  form  two  trunks,  the 
descending  aorta.  The  aortic  arches  pass  around  the  pharynx  between  the  gill 
clefts  in  the  branchial  arches.  The  arrangement  is  like  that  of  the  adult  fish 
which  has  its  gill  slits,  branchial  arches  and  aortic  arches  to  supply  the  gills. 
The  descending  aortas  run  caudad  and  opposite  the  lung  buds  unite  to  form  a 
single  median  dorsal  aorta.  This  in  the  region  of  the  posterior  limb  buds 
divides  into  the  two  umbilical  arteries,  which,  curving  cephalad  and  ventrad, 
enter  the  body-stalk  on  each  side  of  the  allantois  and  eventually  ramify  in  the 
villi  of  the  chorion.     The  vitelline  arteries,  large  and  paired  in  the  chick,  are 


Ventricle 


Liver 


R.  vitelline  vein 


Aortic  arches  1-4 

Atrium 

I 'itrllo-umbilical  vein 

L.  umbilical  vein 


Fig.  83. — Ventral  reconstruction  of  a  3.2  mm.  embryo,  showing  vessels  (His). 


Fig.  84. — Lateral  view  of  human  embryo  of  4.2  mm.,  showing  aortic  arches  and  venous  trunk-  His  . 
m.x.  Maxillary  process;  mn,  mandibular  arch;  d.C,  common  cardinal  vein;  jv..  anterior  cardinal  vein; 
c.v.,  posterior  cardinal  vein;  ;.;..  vitelline  vein;  u.a.,  umbilical  artery;  u.v  ,  umbilical  vein ;  all.,  allantois; 
pl.,  placental  attachment  of  body-stalk;  olf..  olfactory  pit;  ot,  otocyst. 


THE  ANATOMY  OF  A  4.2  MM.  HUMAN  EMBRYO  93 

represented  by  a  single  small  trunk  which  branches  on  the  surface  of  the  yolk- 
sac  (Fig.  84).  Compared  with  the  arterial  circulation  of  the  chick  of  lift)'  hours 
the  important  differences  are  (1)  the  development  of  the  fourth  and  the  fifth 
pairs  of  aortic  arches,  and  (2)  the  presence  of  the  chorionic  circulation  by  way 
of  the  umbilical  arteries  in  addition  to  the  vitelline  circulation  found  in  the  fifty- 
hour  chick. 

The  veins  are  all  paired  and  symmetrically  arranged  (Figs.  83  and  84). 
There  are  three  sets  of  them:  (1)  The  blood  from  the  body  of  the  embryo  is 
drained,  from  the  head  end  by  the  anterior  cardinal  veins;  from  the  tail  end  of  the 
body  by  the  posterior  cardinal  veins.  These  veins  on  each  side  unite  dorsal  to 
the  heart  and  form  a  single  common  cardinal  vein  which  joins  the  umbilical  vein 
of  the  same  side.  (2)  Paired  vitelline  veins  in  the  early  stages  of  the  embryo 
drain  from  the  yolk-sac  the  blood  carried  to  it  by  the  vitelline  arteries.  The 
trunks  of  these  veins  pass  back  into  the  body  on  each  side  of  the  yolk-stalk  and 
liver  and  with  the  paired  umbilical  veins  form  a  trunk  which  empties  into  the 
sinus  venosus  of  the  heart.  As  the  liver  develops  it  may  be  seen  (Fig.  83)  that 
the  vitelline  veins  break  up  into  blood  spaces  called  by  Minot  sinusoids.  When 
the  liver  becomes  large  and  the  yolk-sac  rudimentary  the  vitelline  veins  receive 
blood  chiefly  from  the  liver  and  intestine.  (3)  A  pair  of  large  umbilical  veins 
which  drain  the  blood  from  the  villi  of  the  chorion  and  are  the  first  veins  to 
appear.  These  unite  in  the  body-stalk  and,  again  separating,  enter  the  soma- 
topleure  on  each  side.  They  run  cephalad  to  the  septum  transversum  where 
they  unite  with  the  vitelline  veins  to  form  a  common  vitello-umbilical  trunk 
which  empties  into  the  sinus  venosus. 

The  veins  of  this  embryo  are  thus  like  those  of  the  fifty-hour  chick  save  that 
the  umbilical  vessels  are  now  present  and  take  the  place  of  the  allantoic  veins  of  later 
chick  embryos.  The  veins,  like  the  heart  and  arteries,  are  primitively  paired 
and  symmetrically  arranged.  As  development  proceeds  their  symmetry  is 
largely  lost  and  the  asymmetrical  venous  system  of  the  adult  results. 

The  later  stages  of  the  human  embryo  can  not  be  described  in  detail  here. 
The  student  is  referred  to  the  texts  of  Minot,  Keibel,  and  Mall.  Two  embryos 
will  be  compared  with  the  pig  embryos  described  in  Chapter  V.  Figs.  85  and  86 
show  the  human  embryos  described  by  His,  the  age  of  which  was  estimated  by 
him  at  from  two  weeks  to  two  months.  The  figures  show  as  well  as  could  any 
description  the  changes  which  lead  to  the  adult  form  when  the  embryo  may  be 
called  a,  fetus.  The  external  metamorphosis  is  due  principally:  (1)  to  changes 
in  the  flexures  of  the  embryo;  (2)  to  the  development  of  the  face;  (3)  to  the 
development  of  the  external  structure  of  the  sense  organs  (nose,  eye  and  ear); 


THE    FETAL  MEMBRANES   AND   EARLY  HUMAN   EMBRYO 
(3)  U:>Ly^  (4-)   \  (5) 


Jtg.  85. — Embryos  of  His'  NormentafeL     The  embryos  figured  are  of  the  first  month  (Keibel  and  Elze). 

X  5- 
94 


THE  ANATOMY  OF  A  4.2  MM.  HUMAN  EMBRYO 


95 


(4)  to  the  development  of  the  extremities  and  disappearance  of  the  tail.     The 
more  important  of  these  changes  will  be  dealt  with  in  later  chapters. 


Fig.  86. — Embryos  of  the  second  month  from  His'  Xormentafel  (Keibel  and  Elze). 


Age  of  Human  Embryos.— The  ages  of  the  human  embryos  which  have 
been  obtained  and  described  can  not  be  determined  with  certainty,  because  fer- 
tilization does  not  necessarily  follow  directly  after  coitus.     It  has  been  shown 


g6  THE   FETAL   MEMBRANES   AND   EARLY   HUMAN   EMBRYOS 

also  that  ovulation  does  not  always  coincide  with  menstruation  so  that  the 
menstrual  period  cannot  be  taken  as  the  starting  point  of  pregnancy.  In  1868, 
Reichert,  from  studying  the  corpus  luteum  in  ovaries  obtained  during  menstrua- 
tion, concluded  that  ovulation  takes  place  as  a  rule  just  before  menstruation  and 
that  if  the  ovum  is  fertilized  the  next  menstruation  does  not  occur.  Reichert 
then  decided  that  a  human  embryo  of  5.5  mm.,  which  he  had  obtained  from  a 
woman  two  weeks  after  menstruation  failed  to  occur,  must  be  two  weeks,  not 
six  weeks,  old.  His  accepted  Reichert's  views  and  since  then  the  ages  of  embryos 
have  been  estimated  on  this  basis.  According  to  this  view,  Peter's  ovum,  ob- 
tained thirty  days  after  the  last  period,  is  only  three  or  four  days  old.  This  does 
not  agree  at  all  with  what  is  known  of  the  age  of  other  mammalian  embryos. 

From  the  observations  of  Mall  and  obstetricians  of  the  present  day,  we  must 
conclude  that  ovulation  does  not  immediately  precede  menstruation  but  that 
most  pregnancies  take  place  during  the  first  or  second  week  after  the  menstrual 
period.  It  is  therefore  more  correct  to  compute  the  age  of  the  embryo  from  the 
end  of  the  last  menstruation  or,  according  to  Grosser,  from  the  tenth  to  the 
twelfth  day  before  the  first  missed  menstrual  period.  Peter's  embryo  then  would 
be  about  fifteen  days  old.  To  compare  an  embryo  with  one  of  known  age,  the 
length  from  vertex  to  breech  is  usually  taken.  Embryos  of  the  same  age  vary 
greatly  in  size  so  that  their  structure  must  be  taken  into  account.  At  the  present 
time,  the  exact  relation  of  ovulation  to  menstruation  is  not  known  nor  the  exact 
time  required  for  the  fertilized  ovum  to  reach  the  uterus.  The  computed  age 
of  the  embryo  can  be  thus  only  approximate. 

The  period  of  gestation  of  the  human  fetus  is  usually  computed  from  the 
beginning  of  the  last  menstrual  period.  Forty  weeks  or  two  hundred  and  eighty 
days  is  the  time  usually  allowed.  As  some  women  menstruate  once  or  more 
often  after  becoming  pregnant  this  is  not  a  certain  basis  for  computation. 

The  following  are  the  estimated  ages,  lengths,  and  weights  of  human  em- 
bryos according  to  Mall,  Schroeder,  and  Fehling: 

Length  in  Weight 

Age  Millimeters  in  Grams 

Eighteen  to  twenty-one  days 0.5 

Twenty-four  to  thirty  days 2.5 

Thirty-one  to  thirty-five  days 5-5 

Thirty-eight  to  forty-two  days no 

Fifty  days 20.0 

Second    lunar  month 30.0 

Third      lunar  month 7°-°~  ^o-0  2° 

Fourth    lunar  month 180.0  120 

Fifth       lunar  month 250.0  285 

Sixth        lunar  month 3x5-°  °35 

Seventh  lunar  month 37°°  1220 

Eighth    lunar  month 425-°  1700 

Ninth     lunar  month 470.0  2240 

Tenth     lunar  month 5°°.o  325° 


CHAPTER  V 


THE  STUDY  OF  SIX  AND  TEN  MILLIMETER  PIG  EMBRYOS 


A.    THE  ANATOMY  OF  A  6  MM.  PIG  EMBRYO 
In  its  early  stages  the  pig  embryo  is  flattened  out  on  the  surface  of  the  yolk- 
sac  like  a  chick  embryo  (Fig.  87),  but  as  the  head-  and  tail-folds  elongate  the  body 
becomes  flexed  and  twisted  spirally,  making  it  difficult  to  study.     In  embryos 
5  to  7  mm.  long  the  twist  of  the 
body  begins  to  disappear  and  its 
structure  may   be   seen   to   better 
advantage. 

External  Form  of  6  mm.  Em- 
bryo.— When  compared  with  the 
form  of  the  4  mm.  human  embryo, 
its  marked  difference  is  the  convex 
dorsal  flexure  which  brings  the 
head  and  tail  regions  close  together 
(Fig.  88) .  The  flexure  at  the  mesen- 
cephalon forms  an  acute  angle  and 
there  is  a  marked  neck  or  cervical 
flexure.  As  a  result,  the  head  is 
somewhat  triangular  in  form.  The 
body  is  bent  dorsad  in  an  even 
convex  curve  and  the  tail  is  flexed 
sharply  dorsad  and  to  the  right 
side.  Lateral  to  the  dorsal  line 
may  be  seen  the  segments,  which 
become  larger  and  more  differen- 
tiated as  we  go  from  tail  to  head.  At  the  tip  of  the  head  a  shallow  depression 
marks  the  anlage  of  the  olfactory  pit.  The  lens  vesicle  of  the  eye  is  open  to  the 
exterior.  Caudal  to  the  eyes  at  the  sides  of  the  head  are  four  branchial  arches 
separated  by  three  grooves,  the  branchial  clefts.  The  fourth  arch  is  partly  con- 
cealed in  a  triangular  depression,  the  cervical  sinus  (see  Fig.  92).  The  first,  or 
7  97 


Fig.  87. — Pig  embryos  04)  of  seven  and  (B)  of 
eleven  primitive  segments,  dorsal  view,  with  amnion 
cut  away  (Keibel,  Normentafel).     X  20. 


98 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 


mandibular  arch,  forks  ventrally  into  two  processes,  a  smaller  maxillary  and  a 
larger  mandibular  process,  and  the  latter  with  its  fellow  forms  the  mandible  or 
lower  jaw.  The  position  of  the  mouth  is  indicated  by  the  cleft  between  these 
processes.     The  groove  between  the  eye  and  the  mouth  is  the  lacrymal  groove. 

The  second  or  hyoid  arch  is  separated  from  the  mandibular  arch  by  a  hyo- 
mandibular  cleft  which  persists  as  the  external  auditory  meatus.  About  the  dorsal 
end  of  the  cleft  develops  the  external  ear. 

The  heart  is  large  and  through  the  transparent  body  wall  may  be  seen  the 
dorsal  atrium  and  ventral  ventricle.     Caudal  to  the  heart,  a  convexity  indicates 


Cephalic  flexure 


Olfactory  pit 
Yo/K-Sac 


Maxillary  process  ^^^ 


process 
Br.  arch  II 
Br.  arch  III 


Cervical  sinus 

Alrium  of  heart 


Fig.  88. — Pig  embryo  of  6  mm.,  viewed  from  the  left  side.     The  amnion  has  been  removed  and  its  cut 

edge  is  shown  in  the  figure.     X  12. 


the  position  of  the  liver.  Dorsal  to  the  liver  is  the  bud  of  the  anterior  extremity, 
now  larger  than  in  the  4  mm.  human  embryo.  Extending  caudal  to  the  anlage 
of  the  upper  extremity,  a  curved  convexity  indicates  the  position  of  the  right 
mesonephros.  At  its  caudal  end  is  the  bud  of  the  lower  limb.  The  amnion  has 
been  dissected  away  along  the  line  of  its  attachment  ventral  to  the  mesonephros. 
There  is  as  yet  no  distinct  umbilical  cord  and  a  portion  of  the  body-stalk  is  at- 
tached to  the  embryo. 

As  the  term  of  its  development  is  shorter,  a  young  pig  embryo  is  somewhat 
precocious  in  its  development  as  compared  with  a  human  embryo  of  the  same 
se  ("Fig.  89).     In  a  human  embryo  7  mm.  long  the  head  is  larger,  the  tail  shorter. 


I.ATKRAI.    DISSF.i    HON    OF     Till-.    VISCKRA 


99 


The  cervical  flexure  is  more  marked,  the  olfactory  pits  larger  and  deeper.  The 
liver  is  more  prominent,  the  tnesonephros  and  segments  less  so  than  in  the 
6  mm.  pig. 

Lateral  Dissection  of  the  Viscera 
To  understand  the  sectional  anatomy  of  an  embryo,  a  study  of  dissections 
and  reconstructions  is  essential.     For  methods  of  dissection  see  p.  146,  Chapter 


Myelencephiilun 


Spinal  cord 


Cervical  seg- 
ment 8 


Future  milk 
line 


Thoracic  seg- 
ment 12 


Metencephalon 


Mesencephalon 


— Diencephalon 


Yolk-sac  and 
umbilical  cord 


Lumbar  segment  5 

Fig.  89. — A  human  embryo  7  mm.  long,  viewed  from  the  right  side  (Mall  in  Kallmann's  Hand- 
atlas).  /,  77,  III,  branchial  arches  1,  2  and  3;  H,  Hi,  heart;  L,  liver;  L ',  otic  vesicle;  R,  olfactory 
placode;   Tr,  semi-lunar  ganglion  of  trigeminal  nerve. 


VI.  Before  studying  sections,  the  student  should  become  as  well  acquainted 
as  possible  with  the  anatomy  of  the  embryo  and  compare  each  section  with  the 
figures  of  reconstructions  and  dissections. 

Nervous  System. — Fig.  90  shows  the  central  nervous  system  and  viscera 
exposed  on  the  right  side  of  a  5.5  mm.  embryo.  The  ventro-lateral  wall  of  the 
head  has  been  left  intact  with  the  lens  cavity,  olfactory  pit,  and  portions  of  the 
maxillary  and  mandibular  processes,  second  and  third  branchial  arches  and 


IOO 


THE    STUDY   OF    SIX  AND   TEN   MILLIMETER  PIG  EMBRYOS 


cervical  sinus  (see  Fig.  88).  The  brain  is  differentiated  into  the  five  regions, 
telencephalon,  diencephalon,  mesencephalon,  metencephalon  and  myelencephalon. 
The  spinal  cord  is  cylindrical  and  gradually  tapers  off  to  the  tail.  The  anlages 
of  the  cerebral  and  spinal  ganglia  and  the  main  nerve  trunks  are  shown.  The 
oculomotor  nerve  begins  to  appear  from  the  ventral  wall  of  the  mesencephalon. 
Ventro-lateral  to  the  metencephalon   and   myelencephalon  occur  in  order  the 

Sup.  gang.  n.  Q    Otocyst 

Acustic  ganglion 

Geniculate  gang.  n.  7 

Semilunar  gang.  n.  5 

Metencephalon 

Mesencephalon 


Jugular  gang.  n.  10 
Gang,  nodosum  n.  10 
V.  accessorius 


Gang.  Froriep 
Gang.  cerv.  i 
N.  hypoglossus 
Cervical  sinus 

Atrium 

Ventral  lobe  liver 
Dorsal  lobe  liver 

Thoracic  gang.  1 
Mesonephros 

Small  intestine 


N.  oculomotorius 

Diencephalon 

Petrosal  gang. 
n.  g 

Lens  opening 
Olfactory  pit 
Telencephalon 
Yolk-sac 
Allantoic 

Ventricle 
Allantoic  stalk 


Hind-gut 

Fig.  90. — Dissection  of  a  5.5  mm.  pig  embryo,  showing  the  nervous  system  and  viscera  from  the  right 

side.     X  18. 


semilunar  ganglion  and  three  branches  of  the  trigeminal  nerve,  the  geniculate 
ganglion  and  nerve  trunk  of  the  n.  facialis,  the  ganglionic  anlage  of  the  n.  acusticus 
and  the  otocyst.  It  will  be  observed  that  the  nerve  trunks  are  arranged  with 
reference  to  the  branchial  arches  and  clefts.  Caudal  to  the  otocyst  a  continuous 
chain  of  cells  extends  lateral  to  the  neural  tube  into  the  tail  region.  Cellular 
enlargements  along  this  neural  crest  represent  developing  cerebral  and  spinal 


LATERAL   DISSECTION   OF   THE   VISCERA  IOI 

ganglia.  They  are  in  order  the  superior  or  root  ganglion  of  the  glossopharyngeal 
nerve  with  its  distal  petrosal  ganglion;  the  ganglion  jugulare  and  distal  ganglion 
nodosum  of  the  vagus  nerve;  the  ganglionic  crest  and  proximal  portion  of  the 
spinal  accessory  nerve;  and  the  anlage  of  Froriep's  ganglion,  an  enlargement  on 
the  neural  crest  just  cranial  to  the  first  cervical  ganglion.  Between  the 
vagus  and  Frojiep's  ganglion  may  be  seen  the  numerous  root  fascicles  of  the 
hypoglossal  nerve,  which  take  their  origin  along  the  ventro-lateral  wall  of  the 
myelencephalon  and  unite  to  form  a  single  trunk.  The  posterior  roots  of  the 
spinal  ganglia  are  very  short;   their  anterior  or  ventral  roots  are  not  shown. 

The  position  of  the  heart  with  its  ventricle,  atrium  and  sinus  venosus  are 
shown.  The  liver  is  divided  into  a  small  dorsal  and  a  large  ventral  lobe.  The 
fore-gut  emerges  from  between  the  liver  lobes  and  curves  ventrad  to  the  yolk- 
stalk  and  sac.  The  hind-gut  is  partly  hidden  by  the  fore-gut;  it  makes  a  U- 
shaped  bend  from  the  yolk-stalk  to  the  caudal  region.  The  gut  is  attached  to 
the  dorsal  body  wall  by  a  double  layer  of  splanchnic  mesoderm  which  forms  the 
mesentery.  The  long  slender  mesonephros  lies  ventral  to  the  spinal  cord  and 
curves  caudad  from  a  point  opposite  the  eighth  cervical  ganglion  to  the  tail  re- 
gion. The  cranial  third  of  the  mesonephros  is  widest  and  its  size  diminishes 
tailwards.  Between  the  yolk-sac  and  the  tail  the  allantois  is  seen,  its  stalk 
curving  around  from  the  ventral  side  of  the  tail  region. 

Digestive  Canal. — The  arrangement  of  the  viscera  may  be  seen  in  median 
sagittal  and  ventral  dissections  (Figs.  91  and  92),  also  in  the  reconstruction 
shown  in  Fig.  100.  The  mouth  lies  between  the  mandible,  the  median  nasal 
process  of  the  head,  and  the  maxillary  processes  at  the  sides.  The  diverticulum 
of  the  hypophysis,  flattened  cephalo-caudad  and  expanded  laterad,  extends  along 
the  ventral  wall  of  the  fore-brain  (Fig.  99).  Near  its  distal  end,  the  wall  of  the 
brain  is  thickened  and  later  the  posterior  lobe  of  the  hypophysis  will  develop 
from  the  brain  wall  at  this  point. 

The  pharynx  is  flattened  dorso-ventrally  and  is  widest  near  the  mouth.  Its 
lateral  dimension  narrows  caudad,  and  opposite  the  third  branchial  arch  it  makes 
an  abrupt  bend,  a  bend  which  corresponds  to  the  cervical  flexure  of  the  embryo's 
body  (Figs.  99  and  100).  In  the  roof  of  the  pharynx  just  caudal  to  Rathke's 
pocket  is  the  somewhat  cone-shaped  pouch  known  as  SeesseVs  pocket,  which  may 
be  interpreted  as  the  blind  cephalic  end  of  the  fore-gut.  The  lateral  and  ven- 
tral walls  of  the  pharynx  and  oral  cavity  are  shown  in  Fig.  93.  Of  the  four  arches 
the  mandibular  is  the  largest  and  a  groove  partly  separates  the  processes  of  the 
two  sides.     Posterior  to  this  groove  and  extending  in  the  median  line  to  the 


102 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER  PIG   EMBRYOS 


hyoid  arch  is  a  triangular  rounded  elevation,  the  tuberculum  impar,  which  later 
forms  a  part  of  the  tongue.  At  an  earlier  stage  the  median  thyreoid  anlage  grows 
out  from  the  mid-ventral  wall  of  the  pharynx  just  caudal  to  the  tuberculum 
impar.  The  ventral  ends  of  the  second  arch  fuse  in  the  mid-ventral  line  and 
form  a  prominence,  the  copula.     This  connects  the  tuberculum  impar  with  a 


Anlage  of 
tongue 


R.  atrium 


Esophagus 

Interatrial 

foramen 

Lung  bud 

Stomach 


Hepatic 
diverticulum 


Ventral 
pancreas 


Cranial  limb 
intestine 


Genital  ridge 


Pharynx 


Metamere  4        Rathke's  pocket 


Isthmus 

Mesencephalon 
Diencephalon 

-  Bulbus  cordis 
Telencephalon 
Ventricle 

Septum  trans- 
versum 

Liver 

Yolk-sac 

Allantois 

Tail-gut 

Cloaca 

Metanephros 


Spinal  cord  /  I        Caudal  limb  of  intestine 

Mesonephros  Mesonephric  duct 

Fig.  91. — Median  sagittal  dissection  of  a  pig  embryo  of  6  mm.,  to  show  viscera  and  neural  tube.     X  18. 


rounded  tubercle  derived  from  the  third  and  fourth  pairs  of  arches,  the  anlage 
of  the  epiglottis.  Its  cephalic  portion  forms  the  root  of  the  tongue  (compare 
Fig.  151  A  and  B).  Caudal  to  the  epiglottis  are  the  arytenoid  ridges  and  a  slit 
between  them,  the  glottis,  leads  into  the  trachea. 

The  branchial  arches  converge  caudad  and  the  pharynx  narrows  rapidly 
before  it  is  differentiated  into  the  trachea  and  esophagus  (Figs.  99  and  100). 
Laterally  and  ventrally  between  the  arches  are  the  four  paired  outpocketings  of 


LATERAL   DISSECTION   OF   THE    VISCERA 


IO3 


the  pharyngeal  pouches.  The  pouches  have  each  ;i  dorsal  and  ventral  diverti- 
culum (Fox,  Thyng).  The  dorsal  diverticula  are  large  and  wing-like  (Fig.  99), 
meet  the  ectoderm  of  the  gill  clefts,  fuse  with  it  and  form  the  closing  plates. 
Between  the  ventral  diverticula  of  the  third  pouch  lies  the  median  thyreoid 
anlage.     The  fourth  pouch  is  much  smaller  than  the  others.     Its  dorsal  diverticu- 


Eye 
Maxillary  process 

Month 

Br.  arch  3 
Br.  arch  4 

Upper  limb  bud 

Hepatic  diverticulum 

Yolk-sac 

Body-stalk 

Allanlois 

Umbilical  artery 

Mesonephric  duct 


Ironlo-nasal  process 

Olfactory  pit 

Mandibular  process 
Br.  arch  2 

Aortic  bulb 

Trachea 
Lung  bud 

Stomach 

Cephalic  loop  of  intestine 
Mesonephros 
Mesonephric  duct 
Caudal  loop  of  intestine 

Lower  limb  bud 


'Rectum 
Dorsal  aorta  and  umbilical  artery 


Fig.  92. — A  ventral  dissection  of  a  6  mm.  pig  embryo.     The  head  has  been  bent  dorsally.     Br.  arch  2, 
j  and  4,  branchial  arches,  2,  3  and  4. 


lum  just  meets  the  ectoderm,  its  ventral  portion  is  small,  tubular  in  form  and  is 
directed  parallel  to  the  esophagus  (Fig.  99). 

The  groove  on  the  floor  of  the  pharynx  caudal  to  the  epiglottis  is  continu- 
ous with  the  tracheal  groove.  More  caudally  opposite  the  atrium  of  the  heart 
the  trachea  has  separated  from  the  esophagus.  The  trachea  at  once  bifurcates  to 
form  the  primary  bronchi,  and  the  anlages  of  the  lungs.  The  lungs  consist  merely 
of  the  dilated  ends  of  the  bronchi  surrounded  by  a  layer  of  splanchnic  mesoderm. 
They  bud  out  laterally  on  each  side  of  the  esophagus  near  the  cardiac  end  of  the 


104  THE    STUDY   OF   SIX  AND    TEN   MILLIMETER  PIG  EMBRYOS 

stomach,  and  project  into  the  pleural  ccelom.  The  esophagus  is  short  and  widens 
dorso-ventrally  to  form  the  stomach.  The  long  axis  of  the  stomach  is  nearly 
straight,  but  its  entodermal  walls  are  flattened  together  and  it  has  revolved  on 
its  long  axis  so  that  its  dorsal  border  lies  to  the  left,  its  ventral  border  to  the  right, 
as  seen  in  transverse  section  (Fig.  106). 

Caudal  to  the  pyloric  end  of  the  stomach,  and  to  its  right  is  given  off  from 
the  duodenum  the  hepatic  diverticulum.  Its  opening  into  the  gut  is  seen  in  the 
ventral  dissection  (Fig.  92).  The  hepatic  diverticulum  is  a  sac  of  elongated  oval 
form  which  later  gives  rise  to  the  gall  bladder,  cystic  duct  and  common  bile  duct. 
It  is  connected  by  several  cords  of  cells  with  the  trabecular  of  the  liver. 

The  liver  is  divided  incompletely  into  four  lobes,  a  small  dorsal  and  a  large 
ventral  lobe  on  each  side  (Figs.  90  and  107).  The  lobation  does  not  show  in  a 
median  sagittal  section.     The  pancreas  is  represented  by  two  outgrowths.     The 


Lateral  Ungual  anlage- 


i 


Tuberculinn  impar '~--w_., ~-*-~  — j Branchial  arch  II 


Arytenoid  ridge 


Branchial  arch  I 


Epiglottis \f 7^>S5i  "s*dL        S^m Branchial  arch  III 

*gjr  jfl  \  '  Branchial  arch  71' 


Glottis 


Fig.  93. — Dissection  of  the  tongue  and  branchial  arches  of  a  7  mm.  pig  embryo,  seen  in  dorsal  view. 

ventral  pancreas  takes  origin  from  the  hepatic  diverticulum  near  its  attachment 
to  the  duodenum.  It  grows  to  the  right  of  the  duodenum  and  ventrad  to  the 
portal  vein.  The  dorsal  pancreas  takes  origin  from  the  dorsal  side  of  the  duo- 
denum caudal  to  the  hepatic  diverticulum  and  grows  dorsally  into  the  substance 
of  the  gastric  mesentery  (Figs,  ioo  and  108).  It  is  larger  than  the  ventral  pan- 
creas, and  its  posterior  lobules  grow  to  the  right  and  dorsal  to  the  portal  vein  and 
in  later  stages  anastomose  with  the  lobules  of  the  ventral  pancreas. 

The  intestine  of  both  fore-gut  and  hind-gut  has  elongated  and  curves  ven- 
trally  into  the  short  umbilical  cord  where  the  yolk-stalk  has  narrowed  at  its  point 
of  attachment  to  the  gut.  As  the  intestinal  tube  grows  ventrally,  the  layers  of 
splanchnic  mesoderm  which  attach  it  to  the  dorsal  body  wall  grow  at  an  equal 
rate  and  persist  as  the  mesentery. 

The  cloaca,  a  dorso-ventrally  expanded  portion  of  the  hind-gut  gives  off 


LATERAL   DISSECTION   OF   THE    VISCERA  105 

cephalad  and  ventrad  the  allantoic  stalk.  This  is  at  first  a  narrow  tube  but  soon 
expands  into  a  vesicle  of  large  size,  a  portion  of  which  is  seen  in  Fig.  90.  Dorso- 
laterad  the  cloaca  receives  the  primary  excretory  {Wolffian  ducts).  The  hind-gut 
is  continued  into  the  tail  as  the  tail-gut  (post-anal  gut)  which  dilates  at  its  ex- 
tremity as  in  the  7.8  mm.  pig  described  by  Thyng.  The  mid-ventral  wall 
of  the  cloaca  is  fused  to  the  adjacent  ectoderm  to  form  the  cloacal  membrane. 
In  this  region  later  the  anus  arises  (Fig.  100).  The  post-anal  gut  soon  disap- 
pears. 

The  urogenital  organs  consist  of  the  mesonep/iroi,  the  mesonephric  ducts, 
the  anlages  of  the  metanephroi,  the  cloaca  and  the  allantois.  The  form  of 
the  mesonephroi  is  seen  in  Figs.  90  and  92.  Each  consists  of  large  vascular  glom- 
eruli associated  with  coiled  tubules  lined  with  cuboidal  epithelium  and  opening 
into  the  mesonephric  duct  (Figs.  107  and  109).  The  Wolffian  ducts  beginning 
at  the  anterior  end  of  the  mesonephros  curve 
at  first  along  its  ventral,  then  along  its  lateral 
surface.  At  its  caudal  end  each  duct  bends 
ventrad  and  to  the  midline,  where  it  opens 
into  a  lateral  expansion  of  the  cloaca.  Be-  R^tric/e 
fore  this  junction  takes  place,  an  evagination 
into  the  mesenchyme  from  the  dorsal  wall  of 
each    mesonephric    duct    gives    rise   to    the  FlG-  94-— Ventral  and  cranial  sur- 

face  of   the   heart   from   a  6   mm.   pig 

anlages   of    the   metanephroi,   or    permanent       embryo.    X  14. 

kidneys.     A  slight   thickening  of   the   meso- 

thelium  along  the  median  and  ventral   surface  of  each  mesonephros  forms  a 

light-colored  area,  the  genital  fold  (Fig.  91).     This  area  is  pointed  at  either  end 

and  confined  to  the  middle  third  of  the  kidney.     It  is  the  anlage  of  the  genital 

gland  from  which  either  testis  or  ovary  is  developed. 

Blood  Vascular  System. — The  heart  lies  in  the  pericardial  cavity  as  seen  in 
Fig.  91.  The  atrial  region  (Fig.  94),  as  in  the  4.2  mm.  human  embryo,  has 
given  rise  to  two  lateral  sacs,  the  right  and  left  atria.  The  bulbo-ventricular 
loop  has  become  differentiated  into  right  and  left  ventricles  much  thicker  walled 
than  the  atria.  The  right  ventricle  is  the  smaller  and  from  it  the  bulbus  passes 
between  the  atria  and  is  continued  as  the  ventral  aorta.  Viewed  from  the  caudal 
and  dorsal  aspect  (Fig.  95),  the  sinus  venosus  is  seen  dorsal  to  the  atria.  It  opens 
into  the  right  atrium  and  receives  from  the  right  side  the  right  common  cardinal 
vein,  from  the  left  side  the  left  common  cardinal.  These  veins  drain  the  blood 
from  the  body  of  the  embryo.     Caudally  the  sinus  venosus  receives  the  two 


io6 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 


vitelline  veins.     Of  these,  the  left  is  small  in  the  liver  and  later  disappears. 

The  right  vitelline  vein,  now  the  common  hepatic,  carries  most  of  the  blood  to  the 

heart  from  the  umbilical  veins,  from  the  liver  sinusoids,  gut  and  from  the  yolk-sac. 

Transverse  sections  of  the  embryo  through  the  four  chambers  of  the  heart 

show  the  atria  in  communi- 

Bulbus  cordis 


cation  with  the  ventricles 
through  the  atrio-ventricular 
canals  (Fig.  104),  and  the 
sinus  venosus  opening  into 
the  right  auricle.  This  open- 
ing is  guarded  by  the  right 
and  left  valves  of  the  sinus 
venosus.  Septa  incompletely 
separate  the  two  atria  and 
the  two  ventricles.  In  Fig. 
104  the  atrial  septum  {septum 
primum)    appears    complete. 


t.  common 
Cardinal  vein 


Left  ventricle 


R.  Atrium 


R.  vitelline  vein 


.R.Ventricle 


Fig.  95. — Dorsal  and  caudal  view  of  the  heart  from  a  6  mm. 
pig  embryo.     X  21. 


Bulbus  cordis 


In  Fig.  96,  from  a  slightly  smaller  embryo,  it  is  seen  that  the  septum  primum 
grows  from  the  dorsal  atrial  wall  of  the  heart  and  does  not  yet  meet  the 
endocardial  cushions  between  the  atrio-ventricular  canals.  This  opening 
between  the  atria  is  known  as  the  interatrial  foramen.  Before  it  closes, 
another  opening  appears  in  the  septum, 
dorsal  in  position.  This  is  the  foramen 
ovale  and  persists  during  fetal  life.  In 
Fig.  96  these  two  openings  may  be  seen, 
as  may  also  the  dorsal  and  ventral  endo- 
cardial cushions.  The  outer  mesothelial 
layer  of  the  ventricles  has  become  much 
thicker  than  that  of  the  atria.  It  forms 
the  epicardium  and  the  myocardium,  the 
sponge-like  meshes  of  which  are  now  being 
developed. 

The  arteries  begin  with  the  ventral  aorta,  which  takes  origin  from  the  bulbus 
cordis.  From  the  ventral  aorta  are  given  off  five  pairs  of  aortic  arches.  These 
run  dorsad  in  the  five  branchial  arches  (Figs.  99  and  100)  and  join  the  paired 
dorsal  or  descending  aortce.  The  first  and  second  pairs  of  aortic  arches  are  very 
small  and  take  origin  from  the  small  common  trunks  formed  by  the  bifurcation 


Fofamm  oyafe 
WallofLaJrium 
Interatrial  foramen 
Endocardial  cushionr 

Wall  of.l.ventricle 


Fig.  96. — Dissection  of  a  5.5  mm. 
pig's  heart  from  the  left  side,  showing 
the  septum  primum  and  two  interatrial 
foramina.     X  14. 


LATERAL   DISSECTION   OF   THE   VISCERA  107 

of  the  ventral  aorta  just  caudal  to  the  median  thyreoid  gland.  The  fourth  aortic- 
arch  is  the  largest.     From  the  fifth  arch  small  pulmonary  arteries  are  developing. 

al  to  the  first  pair  of  aortic  arches,  the  descending  aortae  are  continued 

ird  into  the  maxillary  processes  as  the  internal  carotids.  Caudal  to  the 
aortic  arches  the  descending  aortae  converge,  unite  opposite  the  cardiac  end  of 
the  stomach  and  form  the  median  dorsal  aorta.  From  this  vessel  and  from  the 
mding  aorta?  paired  dorsal  intersegmental  arteries  arise.  From  the  seventh 
pair  of  these  arteries  (the  first  pair  to  arise  from  the  median  dorsal  aorta),  there 
are  developed  a  pair  of  lateral  branches  to  the  upper  limb  buds.  These  vessels 
are  the  subclavian  arteries.  From  the  median  dorsal  aorta  there  are  also  given 
off  ventro-lateral  arteries  to  the  glomeruli  of  the  mesonephros,  and  median  ventral 
arteries.  Of  the  latter  the  coeliac  artery  arises  opposite  the  origin  of  the  hepatic 
diverticulum.  The  vitelline  artery  takes  origin  by  two  or  three  trunks  caudal  to 
the  dorsal  pancreas.  Of  these  trunks  the  posterior  is  the  larger  and  persists  as 
the  superior  mesenteric  artery.  Thyng  (Anat.  Record,  vol.  5,  191 1)  has  figured 
three  trunks  of  origin  in  the  7.8  mm.  pig.  These  unite  and  the  single  vitelline 
artery  branches  in  the  wall  of  the  yolk-sac. 

Opposite  the  lower  limb  buds  the  dorsal  aorta  is  divided  for  a  short  distance. 
From  each  division  arises  laterad  three  short  trunks  which  unite  to  form  the  single 
umbilical  artery  on  each  side.  The  middle  trunk  is  the  largest  and  apparently 
becomes  the  common  iliac  artery.  A  pair  of  short  caudal  arteries,  much  smaller 
in  size,  continue  the  descending  aortae  into  the  tail  region. 

The  Veins. — The  vitelline  veins,  originally  paired  throughout,  are  now  repre- 
sented distally  by  a  single  vessel,  which,  arising  in  the  wall  of  the  yolk-sac,  enters 
the  embryo  coursing  cephalad  to  the  intestinal  loop  (Figs.  97,  99  and  100).  Cross- 
ing to  the  left  side  of  the  intestine  and  ventral  to  it,  it  is  joined  by  the  superior 
mesenteric  vein  which  has  developed  in  the  mesentery  of  the  intestinal  loop. 
The  trunk  cranial  to  the  union  of  these  two  vessels  becomes  the  portal  vein. 
It  passes  along  the  left  side  of  the  gut  in  the  mesentery.  Opposite  the  origin  of 
the  dorsal  pancreas  it  gives  off  a  small  branch,  a  rudiment  of  the  left  vitelline 
vein,  which  courses  cephalad  and  in  earlier  stages  connects  with  the  sinusoids 
of  the  liver.  The  portal  vein  then  bends  sharply  to  the  right  dorsal  to  the  duo- 
denum and  as  the  right  vitelline  vein,  passing  between  the  dorsal  and  ventral 
pancreas  to  the  right  of  the  duodenum,  it  soon  enters  the  liver  and  connects  with 
the  liver  sinusoids.  The  portal  trunk  is  thus  formed  by  persisting  portions  of 
both  vitelline  veins,  and  receives  a  new  vessel,  the  superior  mesenteric  vein.  The 
middle  portions  of  the  vitelline  veins  are  connected  with  the  network  of  liver 


ioS 


THE    STUDY   OF   SIX  AND    TEN   MILLIMETER  PIG  EMBRYOS 


sinusoids.  Their  proximal  vitelline  trunks  drain  the  blood  from  the  liver  and 
open  into  the  sinus  venosus  of  the  heart.  The  right  vitelline  trunk  is  much  the 
larger  and  persists  as  the  proximal  portion  of  the  inferior  vena  cava  (for  the  de- 
velopment of  the  portal  vein  see  Chapter  IX) . 

The  umbilical  veins,  taking  their  origin  in  the  walls  of  the  chorion  and  allan- 
toic vesicle,  he  caudal  and  lateral  to  the  allantoic  stalk  and  anastomose  (Figs. 
97  and  99).     Before  the  allantoic  stalk  enters  the  body,  the  umbilical  veins  sepa- 


Spinal  cord 
Ant.  cardinal  vein 

Cervical  sinus 

Pericardial  cavity 

Atrial  junction  sinus-, 

venosus 

Sinus  venosus 

Right  vitelline  vein 

Liver 

Large  venous  sinusoid 

of  Liver 

Hepatic  diverticulum 

{cut) 

Yolk-stalk 

Portal  vein 

Cephalic  limb 
intestinal  loop 

Right  umbilical  vein 
Vitelline  artery 


Nolochord 

Ant.  cardinal  vein 

Pharynx 

Pericardial  cavity 

Left  common  cardinal 

vein 

Left  horn  of  sinus 

venosus 

Left  vitelline  vein 

Ductus  venosus 

Ant.  limb  bud 

Inf.  vena  cava 

Dorsal  pancreas 

- — Left  vitelline  vein 

Common  vitelline  vein 

Left  umbilical  vein 

Sup.  mesenteric  vein 


sLeft  umbilical  artery 
Caudal  limb  •' 
intestinal  loop 
Right  umbilical  artery 

Dorsal  aorta ' 

Fig.  97. — Reconstruction  in  ventral  view  of  a  6  mm.  pig  embryo  to  show  the  vitelline  and  umbilical 
veins,  the  latter  opened  (original  drawing  by  Mr.  K.  L.  Vehe). 


Post,  limb  bud 
Spinal  cord 


rate  and  run  lateral  to  the  umbilical  arteries.  The  left  vein  is  much  the  larger. 
Both,  after  receiving  branches  from  the  posterior  limb  buds  and  from  the  body 
wall,  pass  cephalad  in  the  somatopleure  at  each  side.  Their  course  is  first 
cephalad,  then  dorsad,  until  they  enter  the  liver.  The  left  vein  enters  a  wide 
channel,  the  ductus  venosus,  which  carries  its  blood  through  the  liver,  thence  to 
the  heart  by  way  of  the  right  vitelline  trunk.  The  right  vein  joins  a  large  sinu- 
soidal continuation  of  the  portal  vein  in  the  liver.  This  common  trunk  drains 
into  the  ductus  venosus. 


LATERAL  DISSECTION   OF    THE    VISCERA 


IO9 


The  anterior  cardinal  veins  are  formed  by  the  plexus  of  veins  on  each  side 
of  the  head  which  are  drained  by  two  trunks  (Figs.  98  and  99).  These  extend 
caudad  and  lie  lateral  to  the  ventral  portion  of  the  myelencephalon.  Each  an- 
terior cardinal  vein  receives  branches  from  the  sides  of  the  myelencephalon,  then 
curves  ventrad,  is  joined  by  the  linguo-facial  vein  from  the  branchial  arches  and 
at  once  unites  with  the  posterior  cardinal  of  the  same  side  to  form  the  common 
cardinal  vein.     This,  as  we  have  seen,  opens  into  the  sinus  venosus. 


Spinal  cord 

Anterior  cardinal  vein 

Cervical  sinus 

Pericardial  cavity 

R.  common  cardinal  vein 

Post,  cardinal  vein 
Esophagus 

Large  venous  sinusoid  Liver 

Anterior  limb  bud  —M 

Inf.  vena,  cava   * 

Post.  Post,  cardinal  vein 

Mesonepliros  (cut  surface) 

R.  subcardinal  vein  — 

Venous  sinusoid  on 

dorsum  of  mesonc phros 

Dorsal  aorta 
Notochord 


Xotochord 
Pharynx 

Trachea 

L.  common  cardinal  vein 
Lung 

Liver 

Stomach  (cut  edge) 

Omenta!  bursa 

Mesogastriuni 

Mesonepliros  (cut  surface) 

Capillary  anastomosis  between 
subcardinal  veins 
Vitelline  artery  in  dorsal 
mesentery 

Capillary  anastomosis  between 
subcardinal  veins 

Venous  sinusoid  on  dorsum 
of  mesonepliros 


Spinal  cord 


Fig.  98. — Reconstruction  of  the  cardinal  and  subcardinal  veins  of  a  6  mm.  pig  embryo  showing  the  early 
development  of  the  inferior  vena  cava  (K.  L.  Vehe). 


The  posterior  cardinal  veins  develop  on  each  side  in  the  mesonephric  ridge, 
dorso-lateral  to  the  mesonephros  (Figs.  98  and  99).  Running  cephalad,  they 
join  the  anterior  cardinal  veins.  When  the  mesonephroi  become  prominent,  as 
at  this  stage,  the  middle  third  of  each  posterior  cardinal  is  broken  up  into  sinusoids 
(Minot).  Sinusoids  extend  from  the  posterior  cardinal  vein  veritrally  around 
both  the  lateral  and  medial  surfaces  of  the  mesonephros.  The  median  sinusoids 
anastomose  longitudinally  and  form  the  subcardinal  veins,  right  and  left.     The 


no 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 


subcardinals  lie  along  the  median  surfaces  of  the  mesonephroi,  more  ventrad 
than  the  posterior  cardinals  with  which  they  are  connected  at  either  end.     There 


A.  car.  V.  card 

Meten.  jnt.  ant. 

Ph 


Ph.  R2 


Ph.  R3 


Mesen. 


Ao.  v. 


V.  oph. 


Ch.  d. 


Ao.  desc. 


Med.  sp. 


V.  card. post.  Aa.  mes.  Mes. 

Fig.  99. — Reconstruction  of  7.8  mm.  pig  embryo  showing  veins  and  aortic  arches  from  the  left  side 
(Thyng).  X  15.  Ao.  desc,  descending  aorta;  Ao.  v.,  ventral  aorta;  A.  car.  int.,  internal  carotid 
artery;  Aa.  mes.,  mesonephric  arteries;  A.  pul.,  pulmonary  artery;  At.  d.,  right  atrium;  D.  v.,  ductus 
venosus;  Ph.  P.  1,  2,  3,  4,  pharyngeal  pouches;  5.  v.,  sinus  venosus;  Th.  med.,  thryeoid  gland;  V.  card, 
ant.,  anterior  cardinal  vein;  V.  card.  com.  d.,  right  common  cardinal  vein;  V.  card.  com.  s.,  left  common 
cardinal  vein;  V.  card,  post.,  posterior  cardinal  vein;  V.  hep.  com.,  common  hepatic  vein;  V.  is.,  inter- 
segmental vein;  V.  ling-fac,  linguo-facial  vein;  V.  oph.,  ophthalmic  vein;  V.  p.,  portal  vein;  V.scard. 
d.,  right  subcardinal  vein;  V.  scl.  d.,  right  subclavian  vein;  V.  umb.  d.,  right  umbilical  vein;  Vcn.  d., 
ri^'ht  ventricle. 


is  a  transverse  capillary  anastomosis  between  them,  cranial  and  caudal  to  the 
permanent  trunk  of  the  vitelline  artery.  The  right  subcardinal  is  connected 
with  the  liver  sinusoids  through  a  small  vein  which  develops  in  the  mesenchyme 


TRANSVERSE   SECTIONS 


III 


of  the  plica  venae  cavas  (caval  mesentery)  located  to  the  right  of  the  mesentery 
( Fig.  107).  This  vein  now  carries  blood  direct  to  the  heart  from  the  right  pos- 
terior cardinal  and  right  subcardinal,  by  way  of  the  liver  sinusoids  and  the  right 
vitelline  trunk  (common  hepatic  vein).  Eventually  these  four  vessels  form  the 
unpaired  inferior  vena  cava.  (For  the  development  of  the  inferior  vena  cava  see 
Chapter  IX).     - 


First  aortic  arch     Seessel's  pocket 


Second  aortic  arch 
Pharynx 
Thyreoid 
Third  aortic  arch 


Xolochord 


Fourth  aortic  arch 
102 
Fifth  aortic  arch  and 
pulmonary  artery 
103 
Esophagus    ■ 
104 
Trachea 

105 
R.  lung 
106 


107 

Stomach 

108 

Cccliac  artery 

log 
Ventral  pancreas 
Dorsal  pancreas 
Gall  bladder 

L.  umbilical  vein 

Vitelline  artery 


Isthmus 


Int.  carotid  artery 

■)i 

Pituitary  bodv  (pharyngeal 
lobe) 

Optic  recess 
102 

Telencephalon 
Ventral  aorta 

Bulbus  cordis 

Interventricular  foramen 
L.  horn  of  sinus  venosus 
io5  s-L.  umbilical  vein 

Tail-gut 

Cloaca 

107 
Spinal  cord 
in 
112 
108 

Melanephric  anlage 
iog 
L.  umbilical  artery 


'Anastomosis  between 
dorsal  aorta 

Allantoic  stalk 
L.  dorsal  aorta 
Mesonephric  duct 


Cephalic  limb  of  intestinal  loop 

Dorsal  aorta        I  Artery  to  mesonephros 

Mesentery  Caudal  limb  of  intestinal  loop 
Fig.  100. — Reconstruction  of  a  6  mm.  pig  embryo  in  the  median  sagittal  plane,  viewed  from  the 
right  side.  The  numbered  heavy  lines  indicate  the  levels  of  the  transverse  sections  shown  in  Figs. 
101-112.  The  broken  lines  indicate  the  outline  of  the  left  mesonephros  and  the  course  of  the  left  um- 
bilical artery  and  vein.  The  latter  may  be  traced  from  the  umbilical  cord  to  the  liver  where  it  is  sec- 
tioned longitudinally.     (Original  drawing  and  reconstruction  by  Mr.  K.  L.  Vehe).     X  i63  ■>■ 


Transverse  Sections 

Having  acquainted  himself  with  the  anatomy  of  the  embryo  from  the  study 

of  dissections  and  reconstructions,  the  student  should  examine  serial  sections  cut 

in  the  plane  indicated  by  guide  lines  on  Fig.  100.     Refer  back  to  the  external 

structure  of  the  embryo  (Fig.  88),  to  the  lateral  dissection  of  the  organs  (Fig. 


112 


THE    STUDY    OF    SIX   AND    TEN    MILLIMETER   PIG   EMBRYOS 


90) ,  and  note  the  plane  of  each  section  and  the  structures  which  would  appear  in 
Fig.  100.  Sections  typical  of  certain  regions  should  be  drawn.  The  various 
structures  may  be  recognized  by  referring  to  the  figures  of  sections  in  the  text, 
and  they  should  be  traced  through  the  series  as  carefully  as  time  will  allow. 

Transverse  Section  through  the  Myelencephalon  and  Otocysts  of  a  6  mm.  Embryo 

(Fig.  101). — As  the  head  is  bent  nearly  at  right  angles  to  the  body,  this  section  passes 
lengthwise  through  the  myelencephalon.  The  diencephalon  is  cut  transversely.  The  cellular 
walls  of  the  myelencephalon  show  a  series  of  six  pairs  of  constrictions,  the  neuromeres.     Lateral 


Fourth  ventricle 

Neur.  6 
Gang,  jugular  n.io 
Neur.  5 
Otocyst 
Neur.  4 
Neur.  3 
Neur.  2 

Neur.  1 


Int.  carotid  artery 


Prosencephalon 


Myelencephalon 


Gang,  superior  n.  g 

Ant.  cardinal  vein 
Gang,  acust.  n.  S 
Gang,  geniculat.  n.  7 

Gang,  semilunar. 
Vein 

Vein 


Fig.  ioi. — Transverse  section  through  the  myelencephalon  and  otocysts  of  a  6  mm.  pig  embryo. 
X  26.5.  Ant.  cardinal  vein,  anterior  cardinal  vein;  Gang,  acust.  n.S,  acustic  ganglion  of  acustic  nerve; 
Gang,  geniculat.  n.7,  geniculate  ganglion  of  the  facial  nerve;  Int.  carotid  artery,  internal  carotid  artery; 

Neur.  1,  2,  3,  4,  neuromeres  1,  2,  3,  and  4. 


to  the  fourth  pair  of  neuromeres  are  the  otocysts,  which  show  a  median  outpocketing  at  the 
point  of  entrance  of  the  endolymph  duct.  The  ganglia  of  the  nn.  trigeminus,  facialis,  acus- 
ticus  and  the  superior  ganglion  of  the  glossopharyngeal  nerve  occur  in  order  on  each  side. 
Sections  of  the  anterior  cardinal  vein  and  its  branches  show  on  the  left  side.  Ventral  to  the 
diencephalon  are  sections  of  the  internal  carotid  arteries. 

Passing  along  down  the  series  into  the  pharynx  region,  observe  the  first,  second  and  third 
pharyngeal  pouches.  Their  dorsal  diverticula  come  into  contact  with  the  ectoderm  of  the 
branchial  clefts  and  form  the  closing  plates. 

Transverse  Sections  through  the  Branchial  Arches  and  the  Eyes  (Fig.  102). — 
The  section  passes  lengthwise  through  the  four  branchial  arches,  the  fourth  sunken  in  the 
cervical  sinus.  Dorsad  is  the  spinal  cord  with  the  first  pair  of  cervical  ganglia.  The  pharynx 
is  cut  across  between  the  third  and  fourth  branchial  pouches.     In  its  floor  is  a  prominence, 


TRANSVERSE    SECTIONS 


113 


the  anlage  of  the  epiglottis.  Ventral  to  the  pharynx  the  ventral  aorta  gives  off  two  pairs  of 
vessels.  The  larger  pair  are  the  fourth  aortic  arches  which  curve  dorsad  around  the  pharynx 
to  enter  the  descanting  aorlcc.  The  smaller  third  aortic  arches  enter  the  third  branchial  ur<  hes 
on  each  side.  A  few  sections  higher  up  in  the  series  the  ventral  aorta  bifurcates  and  the  right 
and  left  trunks  thus  formed  give  off  the  first  and  second  pair  of  aortic,  arches.  Craniallv  in 
the  angle  between  their  common  trunks  lies  the  median  thyreoid  anlage.  The  anterior  cardinal 
veins  are  located  lateral  and  dorsal  to  the  descending  aorta?.  The  end  of  the  head  is  cut  through 
the  telencephalon  and  the  optic  vesicles.  On  the  left  side  of  the  figure  the  lens  vesicle  may  be 
seen  still  connected  with  the  ectoderm.  The  optic  vesicle  now  shows  a  thick  inner  and  a  thin 
outer  layer;   these  form  the  nervous  and  pigment  layers  of  the  retina  respectively. 


Spinal  gang. 

Xotochord 

A  nl.  cardinal  vein 

Pharynx 

Phar.  pouch  3 

Aortic  arch  3 

Phar.  pouch  2   ^*sj&& 


Neural  tube 
Myotome 


Lens  of  eye 


Prosencephalon 


Optic  vesicle 


Fig.  102. — Transverse  section  through  the  branchial  arches  and  eyes  of  a  6  mm.  pig  embryo. 
X  26.5.  Disc,  aorta,  descending  aorta;  Br.  arch  2,  3,  4,  branchial  arch  2,  3  and  4;  Phar.  pouch  2,  j, 
pharyngeal  pouches  2  and  3;  A',  aortic  arch  4. 


Transverse  Section  through  the  Tracheal  Groove,  Bulbus  Cordis  and  Olfactory 
Pits  (Fig.  103). — The  ventral  portion  of  the  figure  shows  a  section  through  the  tip  of  the 
head.  The  telencephalon  is  not  prominent.  The  ectoderm  is  thickened  and  slightly  invagi- 
nated  ventro-latcrad  to  form  the  anlages  of  the  olfactory  pits.  These  deepen  in  later  stages 
and  become  the  nasal  cavities.  In  the  dorsal  portion  of  the  section  may  be  seen  the  cervical 
portion  of  the  spinal  cord,  the  notochord  just  ventral  to  it,  the  descending  aortcr,  and  ventro- 
lateral to  them  the  anterior  cardinal  veins.  The  pharynx  now  is  small  with  a  vertical  groove 
in  its  floor.  This  is  the  tracheal  groove  and  more  caudad  it  will  become  the  cavity  of  the 
trachea.  The  bulbus  cordis  lies  in  the  large  pericardial  cavity.  On  either  side  the  section  cuts 
through  the  cephalic  portions  of  the  atria.  These  will  become  larger  as  we  go  caudad  in  the 
series. 

8 


H4 


THE    STUDY    OF    SIX  AND    TEN   MILLIMETER  PIG  EMBRYOS 


Transverse  Section  through  the  Heart  (Fig.  104.) — The  section  passes  through  the 
bases  of  the  upper  limb  buds.  Lateral  to  the  descending  aortse  are  the  common  cardinal  veins. 
The  right  common  cardinal  opens  into  the  sinus  venosus  which  in  turn  empties  into  the  right 
atrium,  its  opening  being  guarded  by  the  two  valves  of  the  sinus  venosus.  The  trachea  has  now 
separated  from  the  esophagus  and  lies  ventral  to  it.  Both  trachea  and  esophagus  are  sur- 
rounded by  a  condensation  of  mesenchyme.  The  myocardium  of  the  ventricles  has  formed 
a  spongy  layer  much  thicker  than  that  of  the  atrial  wall.  An  incomplete  interventricular 
septum  leaves  the  ventricles  in  communication  dorsad.  The  septum  primum  is  complete  in 
this  section  but  higher  up  in  the  series  there  is  an  interatrial  foramen  (see  Fig.  96).  The/or- 
amen  ovale  is  not  yet  formed. 


Spinal  chord 
Notochord^ 


Ant  cardinal 
vein 


Ratrium 


Somatopleure 


Olfactory 
pit. 


Myotome 


Descending 
aorta 


arynx 

Pericardial 
cavity 


Bui  bus  cordis 


Telencephalon 


Fig.  103. — Transverse  section  through  the  bulbus  cordis  and  olfactory  pits  of  a  6  mm.  pig  embryo. 

X  26.5. 


Transverse  Section  through  the  Lung  Buds  and  Septum  Transversum  (Fig. 
105). — The  tips  of  the  ventricles  lying  in  the  pericardial  cavity  still  show  in  this  section.  Dor- 
sally  the  pericardial  cavity  has  given  place  to  the  pleuro-peritoneal  cavity.  Into  this  cavity 
project  ventrad  the  Wolffian  ridges  in  which  the  posterior  cardinal  veins  partly  lie.  Into  the 
floor  of  the  pleuro-peritoneal  cavities  bulge  the  dorsal  lobes  of  the  liver,  embedded  in  mesen- 
chyma.  This  mesenchyma  is  continuous  with  that  of  the  somatopleure,  and  forms  a  complete 
transverse  septum  ventrally  between  the  liver  and  heart.  This  is  the  septum  transversum 
which  takes  part  in  forming  the  ligaments  of  the  liver  and  is  the  anlage  of  a  portion  of  the 
diaphragm.  Passing  through  the  septum  are  the  two  proximal  trunks  of  the  vitelline  veins. 
Projecting  laterally  into  the  pleuro-peritoneal  cavities  are  ridges  of  mesenchyma  covered  by 
splanchnic  mesoderm  in  which  the  lungs  develop  as  lateral  buds  from  the  caudal  end  of  the 
trachea.  The  right  lung  bud  is  shown  in  the  figure.  Between  the  esophagus  and  the  lung  is 
a  crescent-shaped  cavity,  the  end  of  the  lesser  peritoneal  sac. 


TRANSVERSE    SECTIONS 


115 


Transverse  Section  through  the  Stomach  (Fig.  to6). — The  section  passes  through 
the  upper  limb  buds  and  just  caudal  to  the  point  at  which  the  descending  aorta  unite  to  form 
the  median  dorsal  aorta.  As  the  liver  develops  in  early  stages,  it  comes  into  relation  with  the 
plica  venoz  cavee  along  the  dorsal  body  wall  to  the  right  side  of  the  dorsal  mesogastrium.     The 


Myotome 

Descending  aorta 
Esophagus 

Sinus  venosus 

Valve  of 
sinus  Venosus 

R.  atrium 

Atrioventricular 
opening 

Inter  ventricular 
Septum 

R.Ventncle 
Oomalopleure 


Spinal  cord 


L  Common 
Cardinal  Vein 


Jntervenlrlc- 
ular  foramen 

Peri  cardial 
cavity 


Fig.  104. — Transverse  section  through  the  four  chambers  of  the  heart  of  a  6  mm.  pig  embryo.     X  26.5. 


Spinal  ganglion 
Spinal  nerve 

Descending  aorta. 

Pleuro-peri- 

toneal  Cavity 

R.lung  bud 

Rvitelline  vein 
Septum  transversum 

Pericardial 
Cavity 

R.  Ventricle 


Spinal  cord 


Upper  limb  bud 

Post,  cardinal 
Vein 

esophagus 

Dorsal 
lobe  liver 

Lesser  sac 
L.  vitelline 


L.  ventricle 


Fig.  105. —Transverse  section  through  the  right  lung  bud  and  septum  transversum  of  a  6  mm.  pig 

embryo.     X  26.5. 


i6 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 


space  between  the  liver  and  plica  to  the  right,  and  the  stomach  and  its  omenta  to  the  left,  is 
a  caudal  continuation  of  the  lesser  peritoneal  sac.  The  dorsal  wall  of  the  stomach  is  rotated 
to  the  left,  its  ventral  wall  to  the  right.     The  liver  shows  a  pair  of  dorsal  lobes  and  contains 


Spinal  gang. 

Nohchord 

Dorsal  aorta. 

teritoneal  cavity 
Lesser  sac 

Common  hepatic 
Vein  (R.vitel line) 

ff.  ventral  lobe 
liver 

R.  ventricle 


Spinal  cord 

Spinal  nerve 

Post,  card. Vein 
Upper  limb  bud 
Otomach 


L.ventral  lobe 
fiver 


Lventricle 


Fig.  106. — Transverse  section  through  the  stomach  of  a  6  mm.  pig  embryo.     X  26.5. 


Spinal  cord 

Nolochord 
Posf.  card,  vein 

Dorsal  aorta 

inf.  vena  cava. 

Portal  vein 
n.  umbilical  vein 
Hepatic  diverticulum 


Myotome 

Postcard  vein 

Upper  limb  bud 

—  Dorsal  rnesogastrium 
Dorsal  lobe  Liver 

{..Vitelline  \/ein 

I. umbilical  Vein 


Peritoneal  Cavity 
Fig.  107. — Transverse  section  through  the  hepatic  diverticulum  of  a  6  mm.  pig  embryo.     X  26.5. 

large  blood  spaces  and  networks  of  sinusoids  lined  with  endothelium.     Ventral  to  the  liver, 
the  tips  of  the  ventricles  are  seen. 

Transverse    Section  through  the  Hepatic   Diverticulum   (Fig.   107).— The  upper 
limb  buds  are  prominent  in  this  section.     The  mesonephric  folds  show  the  tubules  and  glomeruli 


TRANSVERSE    SECTIONS 


117 


of  the  mcsoncphroi  and  the  posterior  cardinal  veins  arc  connected  with  the  mesonephric  sinu- 
soids. To  the  median  side  of  the  right  mesonephros  shows  the  right  subcardinal  vein.  From  the 
dorsal  attachment  of  the  liver  there  is  continued  down  into  this  section  a  ridge  on  the  dorsal 
body  wall  just  to  the  right  (left  of  figure)  of  the  mesentery.  In  this  ridge  lies  a  small  vein 
which  connects  cranially  with  the  liver  sinusoids,  caudally  with  the  right  subcardinal  vein. 
As  it  later  forms  a  portion  of  the  inferior  vena  cava,  the  ridge  in  which  it  lies  is  termed  the 
plica  vena  cava;  or  caval  mesentery.  The  right  dorsal  lobe  of  the  liver  contains  a  large  blood 
space  into  which  opens  the  portal  vein.  The  duodenum  has  curved  ventral  to  the  position 
occupied  by  the  stomach  in  the  previous  section.  There  is  given  off  from  it  ventrad  and  to 
the  right  the  hepatic  diverticulum.  In  the  sections  higher  up  small  ducts  from  the  liver  tra- 
becular may  be  traced  into  connection  with  it.  In  the  left  ventral  lobe  of  the  liver,  a  large- 
blood  space  indicates  the  position  of  the  left  umbilical  vein  on  its  way  to  the  ductus  venosus. 


Dorsal  aorta 

R.  post,  cardinal 
Vein 

Glomerulus  of 
Mesonephros 

Inf.  vena  cava 


Portal 


vein 


ff.  umbilical 
Vein 


Distal  end 
hepatic  diverticulum 


Spinal  cord 

L.  post,  cardinal 
vein 

Mesonephros 
Upper  limb  bud 

Mesentery 

Dorsal  pancreas 
[..vitelline  Vein 

Duodenum 

L.umbilical  Vein 


Ventral  pancreas 
Fig.  108. — Transverse  section  through  the  dorsal  pancreas  of  a  6  mm.  pig  embryo.     X  26.5. 


Transverse  Section  through  the  Dorsal  Pancreas  (Fig.  108). — At  this  level  the 
upper  limb  buds  still  show,  the  mesenephroi  are  larger  and  marked  by  their  large  glomeruli. 
The  right  posterior  cardinal  vein  is  broken  up  into  mesonephric  sinusoids.  The  vein  in  the  plica 
Venae  cavae  will,  a  few  sections  lower,  connect  with  the  right  subcardinal  vein.  The  anlage  of 
the  dorsal  pancreas  is  seen  extending  from  the  duodenum  dorsad  into  the  mesenchyme  of  the 
mesentery.  It  soon  bifurcates  into  a  dorsal  and  right  lobe,  of  which  the  latter  is  slightly 
lobulated.  Yentro-latcral  to  the  duodenum,  the  anlage  of  the  ventral  pancreas  is  seen  cut 
across.  It  may  be  traced  cephalad  in  the  series  to  its  origin  from  the  hepatic  diverticulum. 
To  the  right  of  the  ventral  pancreas  (left  of  figure)  lies  the  portal  vein  (portion  of  right  vitelline). 
To  the  left  of  the  dorsal  pancreas  is  seen  the  remains  of  the  left  vitelline  vein.  The  ventral 
lobes  of  the  liver  are  just  disappearing  at  this  level.  In  the  mesenchyme  which  connects 
the  liver  with  the  ventral  body  wall  lie  on  each  side  the  umbilical  veins,  the  left  being  the 
larger.  Between  the  veins  is  the  extremity  of  the  hepatic  diverticulum.  The  body  wall  is 
continued  ventrad  to  form  a  short  umbilical  cord. 


n8 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 


Transverse  Section  at  Level  of  the  Origin  of  the  Vitelline  Artery  and 
Umbilical  Arteries  (Fig.  109). — As  the  posterior  half  of  the  embryo  is  curved  in  the  form 
of  a  half  circle,  sections  caudal  to  the  liver,  like  this  one,  pass  through  the  lower  end  of  the 
body  at  the  level  of  the  posterior  limb  buds.  Two  sections  of  the  embryo  are  thus  seen  in  one, 
their  ventral  aspects  facing  each  other  and  connected  by  the  lateral  body  wall.  In  the  dorsal 
part  of  the  section  the  mesonephroi  are  prominent  with  large  posterior  cardinal  veins  lying  dorsal 


Spinal  cord 

Noto  chord 
ff.post  card  Vein 

Dorsal  aorta 

R.sub,  card. vein 

Mesentery 

Cephalic  limb 
of  intestine 

/?.  umbilical  l/em 


Caudal  Jirnb  of 
intestine 

Vein    „ 
Tail  . 

Lower  limb  bud 
mesonephric  duct 

Dorsal  aorta 
Spinal  Cord 


Spina,/  nerve 

Post. card. vein 

Mesonephros 
I. sub, card.  Vein 

Lviteltine  Vein 
-  L.umbilical  l/ein 


went 


Fig.  109. — Transverse  section  of  a  6  mm.  pig  embryo  at  the  level  of  the  origin  of  the  vitelline 
artery.  The  lower  end  of  the  section  passes  through  the  posterior  limb  buds.  X  26.5.  Mes.  tubule, 
mesonephric  tubule;   R.  post.  card,  vein,  right  posterior  cardinal  vein. 


to  them.  The  trunk  of  the  vitelline  artery  takes  origin  ventrally  from  the  aorta.  It  may  be 
traced  into  the  mesentery,  and  through  it  into  the  wall  of  the  yolk-sac.  On  either  side  of  the 
vitelline  artery  are  the  subcardinal  veins,  the  right  being  the  larger.  In  the  mesentery  may  be 
seen  two  sections  of  the  intestinal  loop  (the  small  intestine  being  cut  lengthwise,  the  large  intes- 
tine transversely),  and  also  sections  of  the  vitelline  artery  and  veins.  In  the  lateral  body  walls 
ventral  to  the  mesonephros  occur  the  umbilical  veins.  The  left  vein  is  large  and  cut  length- 
wise.    The  right  vein  is  cut  obliquely  twice. 


TRANSVERSE   SECTIONS 


119 


In  the  ventral  portion  of  the  section,  the  lower  limb  buds  are  prominent  laterally.  A 
large  pair  of  arteries,  the  common  iliacs,  are  given  off  from  the  aorta  and  may  be  traced  into 
connection  with  the  umbilical  arteries.  The  large  intestine  supported  by  a  short  mesentery  lies 
in  the  ccelom  near  the  midline.  On  each  side  are  the  mesonepkric  folds,  here  small  and  each  show- 
ing a  section  of  the  mesonephric  duel  and  a  single  vesicular  anlage  of  the  mesonephric  tubules. 
The  mesonephric  ducts  are  sectioned  as  they  curve  around  from  their  position  in  the  dorsal 
portion  of  the  section. 

Section  through  the  Primitive  Segments  and  Spinal  Cord  (Fig.  no).— This 
section  is  near  the  end  of  the  series  and  as  the  body  is  here  curved  it  is  really  a  longitudinal 
section.  At  the  left  side  of  the  spinal  cord  the  oval  cellular  masses  are  the  spinal  ganglia  cut 
across.     The  ectoderm,  arching  over  the  segments,  indicates  their  position.     Each  segment 

shows  an  outer  dense  layer,  the  cutis  plate,  lying  just  be- 
neath the  ectoderm.  This  plate  curves  lateral  to  the 
spindle  shaped  muscle  plate  which  gives  rise  to  the  volun- 
tary muscle.  Next  comes  a  diffuse  mass  of  mesenchyma. 
the  sclerotome,  which,  eventually,  with  its  fellow  of  the 
opposite   side,   surrounds   the   spinal  cord  and  forms   the 

Spinal  gang. 

Inlerseg- 

mental  artery 

M  itscle  plate 


Cutis  plate 

Sclerotome 
Ectoderm 

Spinal  cord 


Fig.  no. — Transverse  sec- 
tion through  the  primitive  seg- 
ments and  spinal  cord  of  a  6 
mm.  pig  embryo.     X  45- 


Vein 

Rjjmbilicol  artery. 
Tail 


Mesonephric  duct  ^SkBuBEm 


Spinal  cord 


jVotochord 


Fig.  iii. — Transverse  section  through  the  umbilical  vessels,  allan- 
tois  and  cloaca  of  a  6  mm.  pig  embryo.     X  45. 


anlage  of  a  vertebra.  From  it  is  developed  also  connective  tissue.  A  pair  of  spinal  nerves 
and  spinal  ganglia  are  developed  opposite  each  somite,  and  pairs  of  small  vessels  are  seen 
between  the  segments.     These  are  dorsal  intersegmental  arteries. 

Section  through  the  Umbilical  Vessels,  Allantois  and  Cloaca  (Fig.  in). — 
We  have  now  studied  sections  at  various  levels  of  the  6  mm.  embryo  to  near  the  end  of  the  series. 
We  shall  next  examine  sections  through  the  caudal  region  and  study  the  anlages  of  the  uro- 
genital organs.  Owing  to  the  curvature  of  the  embryo,  we  will  now  be  going  cephalad  in  our 
series.  The  first  section  passes  through  the  bases  of  the  limb  buds  at  the  level  where  the 
allantoic  stalk,  curving  inward  from  the  umbilical  cord,  opens  into  the  cloaca.  At  either  side 
of  the  allantoic  stalk  may  be  seen  oblique  sections  of  the  umbilical  arteries  and  lateral  to  these 
the  large  left  and  small  right  umbilical  vein.  The  mesonephric  ducts  occupy  the  mesonephric 
ridges  which  project  into  small  caudal  prolongations  of  the  ccelom.  Midway  between  the 
ducts  lies  the  hind-gut,  dorsal  to  the  cloaca.  The  tip  of  the  tail  is  seen  in  section  to  the 
left  of  the  figure. 


120 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER  PIG  EMBRYOS 


Section  through  the  Anlages  of  the  Metonephroi,  Cloaca  and  Hind-gut  (Fig. 
112). — The  metonephroi  are  seen  as  dorsal  evaginations  from  the  mesonephric  (Wolffian)  ducts 
just  before  their  entrance  into  the  cloaca.  Each  consists  of  an  epithelial  layer  surrounded  by 
a  condensation  of  mesenchyme.  Traced  a  few  sections  cephalad  the  mesonephric  ducts  open 
into  the  lateral  diverticula  of  the  cloaca,  which,  irregular  in  outline,  because  it  is  sectioned 
obliquely,  lies  ventral  to  them  and  receives  dorsad  the  hind-gut.  Caudal  to  the  cloaca  in 
this  embryo,  the  tail  bends  abruptly  cephalad  and  to  the  right.  The  blind  prolongation  of  the 
hind-gut  may  be  traced  out  into  this  portion  of  the  tail  until  it  ends  in  a  sac-like  dilatation. 


n.  umbilical  vein 
n.umbi lied  artery 

Tail 


Mesonephric  duct 
Metonephric  onlag 


Spinal 


Ventral  body  wall 
Lumbilical  artery 
Lumbilical  vein 
Allantoic  stalk; 


Cloaca 
Hind-gut 


Notochord 


Fig.  112. — Transverse  section  through  the  anlages  of  the  metanephroi  of  a  six  mm.  pig  embryo.     X  45. 


B.     THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 
The  study  of  embryos  at  this  stage  is  important  as  they  possess  the  anlages 
of  most  of  the  organs.     The  anatomy  of  a  12  mm.  pig  embryo  has  been  carefully 
studied  and  described  by  Lewis  (Amer.  Jour.  Anal.,  vol.  2,  pp.  211-225,  1903). 

External  Form  (Fig.  113).- -The  head  is  now  relatively  large  on  account  of 
the  increased  size  of  the  brain.  The  third  branchial  arch  is  still  visible  in  the 
embryo,  but  the  fourth  arch  has  sunken  in  the  cervical  sinus;  usually  both  have 
disappeared  at  a  slightly  later  stage.  The  olfactory  pits  form  elongated  grooves 
on  the  under  surface  of  the  head  and  the  lens  of  the  eye  lies  beneath  the  ectoderm 
surrounded  by  the  optic  cup.  The  maxillary  and  mandibular  processes  of  the 
first  branchial  arch  are  larger  and  the  former  shows  signs  of  fusing  with  the  median 
nasal  process  to  form  the  upper  jaw.  Small  tubercles,  the  anlages  of  the  external 
ear  have  developed  about  the  first  branchial  cleft  which  itself  becomes  the 
external  auditory  meatus. 

At  the  cervical  bend  the  head  is  flexed  at  right  angles  with  the  body  bringing 
the  ventral  surface  of  the  head  close  to  that  of  the  trunk  and  it  is  probably  owing 
to  this  flexure  that  the  third  and  fourth  branchial  arches  buckle  inward  to  form 
the  cervical  sinus.     Dorsad  the  trunk  forms  a  long  curve  more  marked  opposite 


THE   ANATOMY   OF   TEN   TO   TWELVE   MM.    PIG   EMBRYOS 


121 


the  posterior  extremities.     The  reduction  in  the  trunk  flexures  is  due  to  the  in- 
creased size  of  the  heart,  liver  and  mesonephroi.     These  organs  may  be  seen 


Myelcnccphalon 


Br 

Hyoid  arch 

Cervical  flexure 

Br.  arch  III 

Cervical  sinus  - 


Upper / 

limb  bud     1 


Milk  line  - — i-i~ 


Mes.  segment 


Cephalic  flexure 
Eye 


—  Maxillary  process 


Mandibular 
process 
Olfactory  pit 


Yolk-sac 


Umbilical  cord 


Lower  limb  bud' 

Fig.  113. — Exterior  of  a  10  mm.  pig  embryo  viewed  from  the  right  side.     X  7.     Br.  arch  III,  branchial 
arch  three;  Br.  cleft  I,  first  branchial  cleft;  mes.  segment,  mesodermal  segment. 


Cervical 
flexure 


Ext.  Ear 


Cephalic 
flexure 


through  the  translucent  body  wall  and  the  position  of  the  septum  transfer  sum 
may  be  noted  between  the  heart  and  the  diaphragm,  as  in  Fig.  115.  The  limb 
buds  are  larger  and  the  umbilical 
cord  is  prominent  ventrad.  Dor- 
sally  the  mesodermal  segments  may 
be  seen  and  extending  in  a  curve 
between  the  bases  of  the  limb  buds 
is  the  milk  line,  a  thickened  ridge 
of  ectoderm  which  forms  the  an- 
lages  of  the  mammary  glands.  The 
tail  is  long  and  tapering.  Between 
its  base  and  the  umbilical  cord  is 
the  genital  eminence  (Fig.  115). 

Human  embryos  of  this  stage 
or  slightly  older,  vary  considerably 
in  size  (Fig.  1 14) .  They  differ  from 
pig  embryos  in  the  greater  size  of 


Yolk-sac 


,'Q    of    12 


Fig.   114. — Exterior  of  a  human  ( 
mm.,  viewed  from  the  right  side,  showing  attachment 
of  amnion  (cut  away)  and  yolk-stalk  and  -sac.     X  5. 


122 


THE    STUDY    OF    SIX    AND    TEN    MILLIMETER   PIG   EMBRYOS 


the  head,  the  shorter  tail,  the  much  smaller  mesonephric  region,  the  longer 
umbilical  cord  and  the  less  prominent  segments.  The  yolk-sac  is  pear-shaped 
with  long  slender  yolk-stalk. 

Central  Nervous  System  and  Viscera. — Dissections  show  well  the  form  and 
relations  of  the  organs  (Figs.  115,  116  and  117).  Directions  for  preparing  dis- 
sections are  given  in  Chapter  VI. 

Metencephalon     N.  trochlearis 
Gang.  n.  5  \         I       Mesencephalon 


Gang.  nn.  7  and  8 

N.  facialis 
Gang,  superior  n.  9      \ 
Gang,  jugular e  n.  10 

Gang,  petrosal  n.  9 

Gang.  Froriep 
Gang,  nodos.  n.  10 

N.  accessorius 

X.  l:\poglossus 

Atrium 

Lung 

Gang.  cerv.  8 

Septum  transversum 

Liver 

Mesonephros 
Gang,  tliorac.  10 


N.  oculomotorius 


Diencephalon 

Ophthalmic  r.  n.  5 
•N.  opticus 

Maxillary  r.  n.  5 
Telencephalon 

Mandibular  r.  n.  5 
Chorda  tymp.  n.  7 

Ventricle 


Umbilical  cord 


Genital  eminence 


Fig.  115. — Lateral  dissection  of  a  10  mm.  pig  embryo,  showing  the  viscera  and  nervous  system  from 
the  right  side.  The  eye  has  been  removed  and  the  otic  vesicle  is  represented  by  a  broken  line.  The 
ventral  roots  of  the  spinal  nerves  are  not  indicated.     X  10.5.     n.,  nerve;  r.,  ramus. 

Brain. — Five  distinct  regions  may  be  distinguished  (Figs.  115  and  117): 
(1)  The  telencephalon  with  its  rounded  lateral  outgrowths,  the  cerebral  hemispheres. 
Their  cavities,  the  lateral  ventricles  communicate  by  the  interventricular  foramen 
with  the  third  ventricle.  (2)  The  diencephalon  shows  a  laterally  flattened  cavity, 
the  third  ventricle.  Ventro-laterally  from  the  diencephalon  pass  off  the  optic 
stalks  and  an  evagination  of  the  mid-ventral  wall  is  the  anlage  of  the  posterior 


THE    WAIuMY    «>F    TEN    TO    TWELVE    MM.    PIG    EMBRYOS 


123 


hypophyseal  lobe.  (3)  The  mesencephalon  is  undivided  but  its  cavity  becomes  the 
cerebral  aqueduct  leading  caudally  into  the  fourth  ventricle.  ( 4  1  The  melencephalon 
is  separated  from  the  mesencephalon  by  a  constriction,  the  isthmus.  Dorso- 
laterally  it  becomes  the  cerebellum,  vent  rally  the  pons.  (5)  The  elongated 
myclcnccphalon  is  roofed  over  by  a  thin  non-nervous  ependymal  layer.  Its  ventro- 
lateral wall  is  ..thickened  and  still  gives  internal  indication  of  the  ncuromcrcs. 
The  cavity  of  the  metencephalon  and  myelencephalon  is  the  fourth  ventricle. 

Cerebral  Nerves.     Of  the  twelve  cerebral  nerves  all  but  the  first  (olfactory) 
and  sixth  (abducens)  are  represented  in  Fig.  115.     For  a  detailed  description 


.  1  ccessory  gang.  1 
Accessory  gong.  2 

Ace.  gang.  3 


r —  Myelencephalon 


Ace.  gang   4 


Cent.  gang.  2 


Gang,  nodosum 
X.  12 


Fig.  116. — Dissection  of  the  head  of  a  15  mm.  pig  embryo  from  the  right  side  to  show  the  accessory 
vagus  ganglia  with  peripheral  roots  passing  to  the  hypoglossal  nerve. 

of  these  nerves  see  Chapter  XII.  (2)  The  optic  nerve  is  represented  by  the  optic 
stalk  cut  through  in  Fig.  115.  (3)  The  oculomotor,  a  motor  nerve  to  the  eye 
muscles,  takes  origin  from  the  ventro-lateral  wall  of  the  mesencephalon.  (4) 
The  trochlear  nerve  fibers,  motor,  to  the  superior  oblique  muscle  of  the  eye,  arise 
from  the  ventral  wall  of  the  mesencephalon,  turn  dorsad  and  cross  at  the  isthmus, 
thus  emerging  on  the  opposite  side.  From  the  myelencephalon  arise  in  order 
(5)  the  trigeminal  nerve,  mixed,  with  its  semilunar  ganglion  and  three  branches, 
the  ophthalmic,  maxillary,  and  mandibular;  (6)  the  ;/.  abducens.  motor,  from  the 
ventral  wall  to  the  external  rectus  muscle  of  the  eye;    (7)  the  n.  facialis,  mixed, 


124  THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 

with  its  geniculate  ganglion  and  its  superficial  petrosal,  chorda  tympani  and  facial 
branches;  (8)  the  n.  acusticus,  sensory,  arising  cranial  to  the  otocyst,  with  its 
acustic  ganglion  and  sensory  fibers  to  the  internal  ear;  (9)  caudal  to  the  otocyst 
the  n.  glossopharyngeus,  mixed,  with  its  superior  and  petrosal  ganglia;  (10) 
the  vagus,  sensory,  with  its  jugular  and  nodose  ganglia;  (11)  accompanying  the 
vagus  the  motor  fibers  of  the  spinal  accessory  which  take  origin  between  the 
jugular  and  sixth  cervical  ganglia  from  the  lateral  wall  of  the  spinal  cord  and 
myelencephalon;  the  internal  branch  of  the  n.  accessorius  accompanies  the  vagus; 
the  external  branch  leaves  it  between  the  jugular  and  nodose  ganglia  and  supplies 
the  sternocleidomastoid  and  trapezius  muscles;  (12)  the  n.  hypoglossus,  motor, 
arising  by  five  or  six  fascicles  from  the  ventral  wall  of  the  myelencephalon,  its 
trunk  passing  lateral  to  the  nodose  ganglion  and  supplying  the  muscles  of  the 
tongue. 

From  the  jugular  ganglion  of  the  vagus  extends  a  nodular  chain  of  ganglion  cells.  These 
have  been  interpreted  as  accessory  vagus  ganglia.  They  may,  however,  be  continuous  with 
Froriep's  ganglion  which  sends  sensory  fibers  to  the  n.  hypoglossus.  In  pig  embryos  of  15  to 
16  mm.  this  chain  is  frequently  divided  into  four  or  five  ganglionic  masses,  of  which  occasionally 
two  or  three  (including  Froriep's  ganglion)  may  send  fibers  to  the  root  fascicles  of  the  hypo- 
glossal nerve.     Such  a  condition  is  shown  in  Fig.  116. 

Spinal  Nerves.  These  have  each  their  spinal  ganglion,  from  which  the  dorsal 
root  fibers  are  developed  (Figs.  115  and  131).  The  motor  fibers  take  origin  from 
the  ventral  cells  of  the  neural  tube  and  form  the  ventral  roots  which  join  the 
dorsal  roots  in  the  nerve  trunk. 

In  Fig.  115  the  heart  with  its  right  atrium  and  ventricle,  the  dorsal  and  ven- 
tral lobes  of  the  liver,  and  the  large  mesonephros  are  prominent.  Dorsal,  and 
somewhat  caudal  to  the  atrium,  is  the  anlage  of  the  right  lung.  The  septum 
transversum  extends  between  the  heart  and  the  liver. 

Pharynx  and  Its  Derivatives. — Dorsally  the  anterior  lobe  of  the  hypophysis 
is  long  and  forks  at  its  end  (Figs.  117  and  118).  In  the  floor  of  the  pharynx  are 
the  anlages  of  the  tongue  and  epiglottis  (Fig.  151  A) .  From  each  mandibular  arch 
arises  an  elongated  thickening  which  extends  caudal  to  the  second  arch.  Be- 
tween, and  fused  to  these  thickenings,  is  the  triangular  tuberculum  impar.  The 
opening  of  the  thyreoid  duct  between  the  tuberculum  impar  and  the  second  arch 
is  early  obliterated.  A  median  ridge,  or  copula,  between  the  second  arches  con- 
nects the  tuberculum  impar  with  the  epiglottis,  which  seems  to  develop  from  the 
bases  of  the  third  and  fourth  branchial  arches.  On  either  side  of  the  slit-like 
glottis  are  the  arytenoid  folds  of  the  larynx.     (For  the  development  of  the  tongue, 


THE    ANATOMY    OF    TEN    TO    TWELVE    MM.    PIG    EMBRYOS 


125 


see  p.  158.)  The  pharyngeal  pouches  are  now  larger  than  in  the  6  mm.  pig  (Fig. 
118).  The  first  pouch  persists  as  the  Eustachian  tube  and  middle  ear  cavity, 
the  closing  plate  between  it  and  the  first  branchial  cleft  forming  the  tympanic 
membrane.  The  second  pouch  later  largely  disappears.  About  it,  develops  the 
palatine  tonsil.     The  third  pouch  is  tubular,  directed  at  right  angles  to  the  pha  rvnx 


MetencephaUm 

Tela  choroidal 


Ncuromcrcs  of  myelencephalon 

Notochord 

Tongue 

Spinal  cord 


Dorsal  Pancreas 


Hepatic  diverticulum 


Duodenum 


L.  genital  fold 

L.  mesonepkros 


Dorsal  aorta 


Mesencephalon 

Diencephalon 

Post,  lobe  hypophysis 

Optic  recess 

Telencephalon 

A  nt.  lobe  hypophysis 

Bulb  us  cordis 

Ventricle 

Yolk-sac 

Septum  transversum 
Yolk-stalk 
Liver 

Caecum 

'Small  intestine 
Allanlois 
Urogenital  sinus 

I  'relcr 
Mesonepkric  duct 


Colon 
Umbilical  artery  (cut  away) 

Metanephros       Rectum 

Fig.  117. — Median  sagittal  dissection  of  a  10  mm.  pig  embryo,  showing  the  brain,  spinal  cord  and  viscera 

from  the  right  side.     X  10.5. 


and  meets  the  ectoderm  to  form  a  " closing  plate."  Median  to  the  plate,  the 
ventral  diverticulum  of  the  third  pouch  is  the  anlage  of  the  thymus  gland.  Its 
dorsal  diverticulum  forms  an  epithelial  body,  or  parathyrcoid.  The  fourth 
pouch  is  smaller  and  its  dorsal  diverticulum  gives  rise  to  a  second  parathyreoid 
body.     Its  ventral  diverticulum  is  a  rudimentary  thymus  anlage.     A  tubular 


126 


THE    STUDY    OP    SIX  AND    TEN   MILLIMETER  PIG  EMBRYOS 


outgrowth,  caudal  to  the  fourth  pouch,  is  regarded  as  a  fifth  pharyngeal  pouch 
in  human  embryos  and  forms  the  post-branchial  body  on  each  side  (see  p.  172). 
The  thyreoid  gland,  composed  of  branched  cellular  cords,  is  located  in  the  mid- 
line between  the  second  and  third  branchial  arches  (Fig.  118). 

Trachea  and  Lungs. —  Caudal  to  the  fourth  pharyngeal  pouches  the  eso- 


Ganq.n.s 


Gang.nnj 

Otbcyst 
Phar.  pouch  J, 
Gancj.juauJare  n- 

Aortic  an 

Phar.  pouch 

Caudal  root 
n  hypoglossus 

Ganyfr 

Aortic  arch*- 
Gany.cervT 

PhaxpouthTJ 
Aortic  arch  5 
R  descend.  Aort, 
Esophagus 
Trachea- 
Vertebral  arte 
Subclavian  artery 

R.Lu/ICJ 

R.  Atrium 

Stomach  I 

Dorsal  pan ere i 

Vitelline  artery 

Ventral  pahcreas  I 
Descending  Jlorti 

tiokihor 

Fig.  118. — Reconstruction  of  a  10  mm.  pig  to  show  the 
side.  The  veins  are  not  indicated.  Broken  lines  indicate 
positions  of  the  limb  buds.     X  10. 


Post  lobe  hypophysis 

nt.  lobe  hypophysis 

Eye 

Phar.  pouch  I 

Maxillary  process 
Thyreoid  gland 
Pulmonary  art: 
Aorta 
Yolksac 
RVentrielt 

Septum 
transversum 

Liver 

diverticulum 
Cloaca. 

Allantois 
Rectum 

Ureter 
'etartephros 
Umbilical  artery 
Mesonephric  duct 
Cephalic  limb,  infest,  loop 
position  of  the  various  organs  from  the  right 
the  outline  of  the  left  mesonephros  and  the 


phagus  and  trachea  separate  and  form  entodermal  tubes  (Figs.  117  and  118).  Be- 
fore the  trachea  bifurcates  to  form  the  primary  bronchi  there  appears  on  its  right 
side  the  tracheal  bud  of  the  upper  lobe  of  the  right  lung.  This  bronchial  bud 
is  developed  only  on  the  right  side  and  appears  in  embryos  of  8  to  9  mm.  Two 
secondary  bronchial  buds  arise  from  the  primary  bronchus  of  each  lung,  and  form 
the  anlages  of  the  symmetrical  lobes  of  each  lung  (Fig.  119). 


CHE    ANATOMY    OP   TEX   TO   TWELVE    MM.    PIG    EMBRYOS 


127 


Esophagus  and  Stomach. — The  esophagus  extends  as  a  narrow  tube  caudal 
to  the  lungs,  where  it  dilates  into  the  stomach.  The  stomach  is  wide  from  its 
greater  to  its  lesser  curvature  and  shows  a  cardiac  diverticulum  (Lewis).  The 
pyloric  end  of  the  stomach  has  rotated  more  to  the  right,  where  it  opens  into  the 
duodenum,  from  which  division  of  the  intestine  develop  the  liver  and  pancreas. 

The  liver,  with  its  four  lobes,  fills  in  the  space  between  the  heart,  stomach 
and  duodenum  (Fig.  117).     Extending  from  the  right  side  of  the  duodenum  along 


Lat.  nasal  process 
Lacrymal  groove 

Maxillary  process 
Mandibular  process 

Cervical  sinus 
Trachea 

Tracheal  lung  bud 

Upper  limb  bud 

Septum  transversum 

Hepatic  diverticulum 

Yolk-sac 

Yolk-stalk 


Allantois 
R.  umbilical  artery 


Olfactory  pit 

Eye 

Median  nasal  process 

Br.  arch  2 

Br.  arch  3 

Br.  arch  4 

L.  lung 

Esophagus 

Stomach 

Mesonephric  duct 
Ventral  pancreas 
Mesonephros 
Cephalic  limb  of  intestine 

Caudal  limb  of  intestine 
Rectum 

Mctanephros 


Lower  limb  bud ' 

Mesonephric  duct  /         Spinal  cord 
Rectum 

Fig.  1 19. — Ventral  dissection  of  a  9  mm.  pig  embryo.     The  head  is  represented  as  bent  dorsally. 


the  dorsal  and  caudal  surface  of  the  liver  is  the  hepatic  diverticulum.  It  lies  to 
the  right  of  the  midline  and  its  extremity  is  saccular.  This  saccular  portion 
becomes  the  gall  bladder.  Several  ducts  connect  the  diverticulum  with  the  liver 
cords.  One  of  these  persists  as  the  hepatic  duct  which  joins  the  cystic  duct  of  the 
gall  bladder.  The  proximal  portion  of  the  diverticulum  becomes  the  common 
bile  duct,  or  ductus  cholcdochus.  The  ventral  pancreas  arises  from  the  common 
bile  duct  near  its  point  of  origin  (Fig.  118).  It  is  directed  dorsad  and  caudad  to 
the  right  of  the  duodenum.     The  dorsal  pancreas  arises  more  caudally  from  the 


128  THE    STUDY    OF    SIX   AND    TEN    MILLIMETER   PIG   EMBRYOS 

dorsal  wall  of  the  duodenum  and  its  larger,  lobulated  body  grows  dorsally  and 
cranially  (Figs.  118  and  135).  Between  the  pancreatic  anlages  courses  the 
portal  vein.  In  the  pig,  the  duct  of  the  dorsal  pancreas  persists  as  the  functional 
duct. 

Intestine. — Caudal  to  the  duodenum,  the  intestinal  loop  extends  well  into 
the  umbilical  cord  (Figs.  117  and  118).  At  the  bend  of  the  intestinal  loop  is  the 
slender  yolk-stalk.  The  cephalic  limb  of  the  intestine  lies  to  the  right,  owing  to 
the  rotation  of  the  loop.  The  small  intestine  extends  as  far  as  a  slight  enlarge- 
ment of  the  caudal  limb  of  the  loop,  the  anlage  of  the  ccecum,  or  blind  gut.  This 
anlage  marks  the  beginning  of  the  large  intestine  (colon  and  rectum).  The 
intestinal  loop  is  supported  by  the  mesentery  which  is  cut  away  in  Fig.  117.  The 
cloaca  is  now  nearly  separated  into  the  rectum  and  urogenital  sinus.  The  cavity 
of  the  rectum  is  almost  occluded  by  epithelial  cells  (Lewis). 

Urogenital  System. — The  mesonephros  is  much  larger  and  more  highly  dif- 
ferentiated than  in  the  6  mm.  embryo  (Figs.  115  and  119).  Along  the  middle 
of  its  ventro-median  surface  the  genital  fold  is  now  more  prominent  (Fig.  117). 
In  a  ventral  dissection  (Fig.  119)  the  course  of  the  mesonephric  ducts  may  be 
traced.  They  open  into  the  urogenital  sinus,  which  also  receives  the  allantoic 
stalk. 

The  metanephros,  or  permanent  kidney  anlage,  lies  just  mesial  to  the 
umbilical  arteries  where  they  leave  the  aorta  (Fig.  118).  Its  epithelial  portion 
derived  from  the  mesonephric  duct  is  differentiated  into  a  proximal  slender  duct, 
the  ureter,  and  into  a  distal  dilated  pelvis.  From  this  grow  out  later  the  calyces 
and  collecting  tubules  of  the  kidney.  Surrounding  the  pelvis  is  a  layer  of  con- 
densed mesenchyma,  or  nephrogenic  tissue,  which  is  the  anlage  of  the  remainder 
of  the  kidney. 

Blood  Vascular  System.— The  Heart. — In  Fig.  120  the  cardiac  chambers  of 
the  right  side  are  opened.  The  septum  primum  between  the  atria  is  perforated 
dorsad  and  cephalad  by  the  foramen  ovale.  The  inferior  vena  cava  is  seen  opening 
into  the  sinus  venosus,  which  in  turn  communicates  with  the  right  atrium  through 
a  sagittal  slit  guarded  by  the  right  and  left  valves  of  the  sinus  venosus.  The  right 
valve  is  the  higher  and  its  dorsal  half  is  cut  away.  The  valves  were  united 
cephalad  as  the  septum  spurium.  The  aortic  bulb  is  divided  distally  into  the 
aorta  and  the  pulmonary  artery,  the  latter  connecting  with  the  fifth  pair  of  aortic 
arches.  Proximally  the  bulb  is  undivided.  The  interventricular  septum  is 
complete  except  for  the  interventricular  foramen,  which  leads  from  the  left  ven- 
tricle into  the  aortic  side  of  the  bulb.     Of  the  bulbar  swellings  which  divide  the 


THE    ANATOMY    OF    TEN    TO    TWELVE    MM.    PIG    EMIiKVoS 


129 


Sept.  n 


Left  valve  si 


Inf.  vena  ca 


bulb  into  aorta  and  pulmonary  trunk,  the  left  joins  the  interventricular  septum, 
while  the  right  extends  to  the  endocardial  cushion.  These  folds  eventually  fuse 
and  the  partition  of  the  ventricular  portion  of  the  heart  is  completed.  The  en- 
docardium at  the  atrio-ventricular  openings  is  already  undermined  to  form  the 
anlages  of  the  tricuspid  and  bicuspid  valves.  From  the  caudal  wall  of  the  left 
atrium  is  given-off  a  single  pulmonary  vein. 

The  Arteries. — As  seen  in  Fig.  118,  the  first  two  aortic  arches  have  dis- 
appeared. Cranial  to  the  third  arch,  the  ventral  aortas  become  the  external 
carotids.  The  third  aortic  arches  and  the  cephalic  portions  of  the  descending 
aorta^  constitute  the  internal  carotid  arteries.  The  ventral  aortae  between  the 
third  and  fourth  aortic  arches  persist  as  the  common  carotid  arteries.  The  de- 
scending aorta)  in  the  same  region  are 
slender  and  eventually  atrophy.  The 
fourth  aortic  arch  is  largest  and  on  the 
left  side  will  form  the  aortic  arch  of 
the  adult.  From  the  right  fourth  arch 
caudad,  the  right  descending  aorta  is 
smaller  than  the  left.  Opposite  the 
eighth  segment,  the  two  aortae  unite 
and  continue  caudally  as  the  median 
dorsal  aorta.  The  fifth  aortic  arches 
(the  sixth  of  human  embryos)  are 
connected  with  the  pulmonary  trunk, 
and  from  them  arise  small  pulmonary 
arteries  to  the  lungs.  Dorsal  interseg- 
mental arteries  arise,  six  pairs  from  the 

descending  aortae,  others  from  the  dorsal  aorta.  From  the  seventh  pair,  which 
arise  just  where  the  descending  aortae  fuse,  the  subclavian  arteries  pass  off  to 
the  upper  limb  buds  and  the  vertebral  arteries  to  the  head.  The  latter  are 
formed  by  a  longitudinal  anastomosis  between  the  first  seven  pairs  of  interseg- 
mental arteries  on  each  side,  after  which  the  stems  of  the  first  six  pairs  atrophy. 

Ventro-lateral  arteries  from  the  dorsal  aorta  supply  the  mesonephros  and 
genital  ridge  (Fig.  118).  Ventral  arteries  form  the  cceliac  artery  to  the  stomach 
region,  the  vitelline  or  superior  mesenteric  artery  to  the  small  intestine,  and  the 
inferior  mesenteric  artery  to  the  large  intestine. 

The  umbilical  arteries  now  arise  laterally  from  secondary  trunks  which 
persist  as  the  common  iliac  arteries. 
9 


Tricuspid 
Valve 


Fig.  120. 


-Heart  of  12  mm.  embryo  dissected 
from  the  right  side. 


13  O  THE    STUDY   OF    SIX  AND    TEN   MILLIMETER  PIG  EMBRYOS 

The  Veins. — The  cardinal  veins  have  been  reconstructed  by  Lewis  in  a  12 
mm.  pig  (Fig.  121).  The  veins  of  the  head  drain  into  the  anterior  cardinal  vein, 
which  becomes  the  internal  jugular  vein  of  the  adult.  After  receiving  the  ex- 
ternal jugular  veins  and  the  subclavian  veins  from  the  upper  limb  buds  the  anterior 
cardinals  open  into  the  common  cardinal  veins  (duct  of  Cuvier) . 

The  posterior  cardinal  veins  arise  in  the  caudal  region,  course  dorsal  to  the 


Fig.  121  A. — Reconstruction  of  a  12  mm.  pig  embryo  to  show  the  veins  and  heart  from  the  left  side. 
For  names  of  parts  see  Fig.  121  B  on  opposite  page  (F.  T.  Lewis).     X  13-5- 

mesonephroi,  and  drain  the  mesonephric  sinusoids.  The  subcardiyial  veins 
anastomose  just  caudal  to  the  origin  of  the  superior  mesenteric  artery  and  the 
posterior  cardinals  are  interrupted  at  this  level.  The  caudal  portion  of  the 
right  posterior  cardinal  vein  now  anastomoses  with  the  right  subcardinal  vein 
and  with  it  forms  a  part  of  the  inferior  vena  cava.  The  proximal  portions  of  the 
posterior  cardinals  open  into  the  common  cardinal  veins  as  in  the  6  mm.  embryo. 


THE   ANATOMY    OF   TEN    TO   TWELVE    MM.    PIG   EMBRYOS 


131 


Of  the  two  subcardinal  veins,  the  right  has  become  very  large  through  its  con- 
nection with  the  right  posterior  cardinal  vein  and  the  common  hepatic  vein,  and 
now  forms  the  middle  portion  of  the  inferior  vena  cava.  For  the  development 
of  this  vein,  see  Chapter  IX. 


Geti 


Fig.  121  B. — Reconstruction  of  a  12  mm.  pig  embryo  to  show  the  veins  from  the  left  side  (Lewis). 
X  13.5.  A.,  umbilical  artery;  Ao.,  aorta;  Au..  right  auricle  ^atrium);  Card.',  Card.",  superior  and  in- 
ferior sections  of  posterior  cardinal  vein;  d.  left  common  cardinal  vein;  D.C.,  right  common  cardinal 
vein;  D.V..  ductus  venosus;  Jug.',  Jug.",  jugular  or  ant.  cardinal  vein;  L.,  liver;  L.s.,  anlage  of  lateral 
sinus;  mx,  transverse  vein;  P.,  pulmonary  artery;  Sc,  subcardinal  vein;  Scl.,  subclavian  vein;  Sis., 
anlage  of  sup.  longitudinal  sinus;  L*m.  d..  right  umbilical  vein;  Yen.,  right  ventricle;  Y.H.C.,  common 
hepatic  vein;  Y.op.,  ophthalmic  vein;  Y.P.,  portal  vein;  X,  anastomosis  between  the  right  and  left 
subcardinal  veins. 


The  Umbilical  Veins  (Figs.  121  and  122)  anastomose  in  the  umbilical  cord, 
separate  on  entering  the  embryo,  and  course  in  the  ventro-lateral  body  wall  of 
each  side  cranially  to  the  ventral  lobe  of  the  liver.  The  left  vein  is  much  the 
larger  and,  after  entering  the  liver,  its  course  is  to  the  right  and  dorsad.  After 
connecting  with  the  portal  vein,  it  continues  as  the  ductus  venosus  and  joins  the 


132 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 


proximal  end  of  the  inferior  vena  cava.  The  smaller  right  umbilical  vein  after 
entering  the  liver  breaks  up  into  sinusoids.  It  soon  atrophies,  while  the  left  vein 
persists  until  after  birth. 

The  Vitelline  Veins. — Of  these  a  distal  portion  of  the  left  and  a  proximal 
portion  of  the  right  are  persistent.  The  left  vitelhne  vein,  fused  with  the  right, 
courses  from  the  yolk-sac  cephalad  to  the  intestinal  loop.  Near  its  dorsal  anas- 
tomosis with  the  right  vein  just  caudal  to  the  duct  of  the  dorsal  pancreas,  it 
receives  the  superior  mesenteric  vein,  a  new  vessel  arising  in  the  mesentery  of  the 


Notochord 
Pharvnx 


R.  ant.  cardinal, 
vein 


Pericardial 

Cai/iry 


Lvitelline 
Vein 

Small  inlesT. 


Sup.  mesenteric 
vein 


Spinal  COrd 

Ant  cardinal  vein 
Esophagus 

Trachea. 

Upper  limb 

Common  Cardinal 
Vein 

Ductus  Venosus 

Liver 

Pyloric  stomach 

Hepatic 

diverticulum 

Dorsal  pancreas 
Duodenum 

Lumbilical  Vein 
Allantois 


R. umbilical  Vein 

/?. umbilical  artery 

Fir;.  122. — Reconstruction  of  a  10  mm.  pig  embryo  to  show  the  umbilical  and  vitelline  veins  from  the 
ventral  side.     X  indicates  sinusoidal  connection  between  left  umbilical  vein  and  portal  vein. 


intestinal  loop.  Cranial  to  its  junction  with  the  sup.  mesenteric  vein,  the  left 
vitelline  with  its  dorsal  anastomosis  and  the  proximal  portion  of  the  right  vitel- 
line vein  form  the  portal  vein,  which  gives  off  branches  to  the  hepatic  sinusoids 
and  connects  with  the  left  umbilical  vein.  For  the  development  of  the  portal 
vein,  see  Chapter  IX. 

Transverse  Sections  of  a  10  mm.  Pig  Embryo 
Figures  are  shown  of  sections  passing  through  the  more  important  regions 
and  should  be  used  for  the  identification  of  the  organs.     The  level  and  plane  of 


TRANSVERSA".    SECTIONS    OF    A    TKN    MM.    PK;    F.MIiRYO 


133 


each  section  is  indicated  by  guide  lines  on  Fig.  123.  The  student  should  compare 
this  with  Figs.  113  and  118,  and  orient  each  section  with  reference  to  the  embryo 
as  a  whole.  Keep  in  mind  the  fact  that  the  transverse  sections  are  drawn  from 
the  cephalic  surface  so  that  the  right  side  of  the  figure  is  the  left  side  of  the  em- 
bryo. 

Transverse  Section  through  the  Eyes  and  Otocysts  (Fig.  124). — The  brain  is 
sectioned  twice,  lengthwise  through  the  myelencephalon,  transversely  through  the  fore-brain. 
The  brain  wall  shows  differentiation  into  three  layers:    (1)  an  inner  epcndymal  layer  densely 


Myelencephalon 

Ganyjup  n  9. 


Metencephalon 


Mesencephalon 


Dicncephalon 


Telencephalon 


Olfactory  pit 


Fig.  123. — Reconstruction  of  a  10  mm.  pig  embryo,  showing  the  chief  organs  of  the  left  side.  The 
numbered  lines  indicate  the  levels  of  transverse  sections  shown  in  the  corresponding  figures  (124-138). 
For  the  names  of  the  various  structures  not  lettered  see  Fig.  118.  X  8.  Gang,  and  n.  access.,  ganglion 
and  n.  accessorius;  Gang.  sup.  n.  p.,  superior  ganglion  of  glossopharyngeal  nerve;  Pulm.  artery,  pul- 
monary artery. 


cellular;  (2)  a  middle  mantle  layer  of  nerve  cells  and  fibers;  (3)  an  outer  marginal  layer  chiefly 
fibrous.  These  same  three  layers  are  developed  in  the  spinal  cord.  A  thin  vascular  layer 
differentiated  from  the  mesenchyma  surrounds  the  brain  wall  and  is  the  anlage  of  the  pia 
mater.  The  myelencephalon  shows  three  neuromeres  in  this  section.  The  telencephalon  is 
represented  by  the  paired  cerebral  hemispheres,  their  cavities,  the  lateral  ventricles,  connecting 
through  the  interventricular  foramina  with  the  third  ventricle  of  the  dicncephalon.  Close  to  the 
ventral  wall  of  the  dicncephalon  is  a  section  of  the  anterior  lobe  of  the  hypophysis  (Rathke's 
pocket).  Lateral  to  the  dicncephalon  is  the  optic  cup  and  lens  vesicle  of  the  eye,  which  are  sec- 
tioned caudal  to  the  optic  stalk.  The  outer  layer  of  the  optic  cup  forms  the  thin  pigment  layer; 
the  inner  thicker  layer  is  the  nervous  layer  of  the  retina.  The  lens  is  now  a  closed  vesicle  dis- 
tinct from  the  overlying  corneal  ectoderm. 

The  large  vascular  spaces  are  the  cavernous  sinuses,  which  drain  by  way  of  the  w.  capitis 


134 


THE    STUDY   OF   SIX  AND    TEN   MILLIMETER   PIG  EMBRYOS 


lateralis  into  the  internal  jugular  veins.  Transverse  sections  may  be  seen  of  the  maxillary 
and  mandibular  branches  of  the  n.  trigeminus;  the  n.  abducens  is  sectioned  longitudinally.  Ven- 
tral to  the  otocyst  are  seen  the  geniculate  and  acustic  ganglia  of  the  mi.  facialis  and  acusticus. 
The  wall  of  the  otocyst  forms  a  sharply  defined  epithelial  layer.  More  cephalad  in  the  series 
the  endolymphatic  duct  lies  median  to  the  otocyst  and  connects  with  it.  Dorsal  to  the  oto- 
cyst the  n.  glossopharyngeus  and  the  jugular  ganglion  of  the  vagus  are  cut  transversely  while 
the  trunk  of  the  n.  accessor ius  is  cut  lengthwise. 

Section  through  the  First   and   Second  Pharyngeal  Pouches   (Fig.   125). — The 
end  of  the  head,  with  sections  of  the  telencephalon  and  of  the  ends  of  the  olfactory  pits,  is  now 


Fourth  ventricle 


Gany.juaulare  n.  10 -M 


CcuiQ.acust.ri.8 


Mandibular  ramus 

77. .5 

Maxi/lary  ramus 
'■n-5 

Ant  lobe  hypophysis 
Lens  vesicle 


Wall  of 
Myelencephalon 


M  accessories 


N.  glossophar- 
yngeus 

Otocyst 

Gang,  genicul.  n.  7 

N.  abducens 

Basilar  artery 

Sinus  cavern. 

Int. carotid- 
art  erv 


Optic  vesicle 


Foramen 
intervenr. 


Third  venlricle  of. 
telencephalon 

Lat.venTricle  of 
telencephalon 

Fig.  124. — Transverse  section  passing  through  the  eyes  and  otocysts  of  a  10  mm.  embryo.     X  22.5. 


distinct  from  the  rest  of  the  section.  The  pharynx  shows  portions  of  the  first  and  second  pharyn- 
geal pouches.  Opposite  the  first  pouch  externally  is  the  first  branchial  cleft.  A  section  of  the 
tuberculum  impar  of  the  tongue  shows  near  the  midline  in  the  pharyngeal  cavity.  The  neural 
tube  is  sectioned  dorsally  at  the  level  of  Froriep's  ganglion.  Between  the  neural  tube  and  the 
pharynx  may  be  seen  on  each  side  the  several  root  fascicles  of  the  n.  hypoglossus,  the  fibers  of 
the  nn.  vagus  and  accessor  ius  and  the  petrosal  ganglion  of  the  n.  glossopharyngeus.  Mesial  to 
the  ganglia  are  the  descetuling  aorta;  and  lateral  to  the  vagus  is  the  internal  jugular  vein. 

Section  through  the  Third  Pharyngeal  Pouches  (Fig.  126). — The  tip  of  the  head 
i=  now  small  and  shows  on  either  side  the  deep  olfactory  pits  lined  with  thickened  olfactory  epi- 


TRANSVERSE    SECTIONS    OF   A   TEN   MM.    PIG   EMBRYO 


135 


thelium.  The  first,  second  and  third  branchial  arches  show  on  cither  side  of  the  section,  the 
third  being  slightly  sunken  in  the  cervical  sinus.  The  dorsal  diverticula  of  the  third  pharyngeal 
pouches  extend  toward  the  ectoderm  of  the  third  branchial  cleft.  The  ventral  diverticula  or 
thymic  anlages  may  be  traced  caudad  in  the  series.  The  floor  of  the  pharynx  is  sectioned 
through  the  epiglottis.  Ventral  to  the  pharynx  are  sections  of  the  third  aortic  arches  and  the 
solid  cords  of  the  median  thyreoid  gland.  Dorsally  the  section  passes  through  the  spinal  cord  and 
first  pair  of  cervical  ganglia.  Between  the  cord  and  pharynx,  named  in  order,  are  tin-  internal 
jugular  veins,  the  hypoglossal  nerve,  and  the  nodose  ganglion  of  the  vagus.     Lateral  to  the  ganglion 


N.accessonus 

Cangfroriep 
Myelencepholm 


Basfa.ra.rt  ► 

Noto chord 

Ganaptfrosal 
Phar  pouch  Z 

Pharpouchl- 
Oral  cavity 

Olfactory  pii 
Telencephalon 


Fig.  125. — Transverse  section  passing  through  the  first  and  second  pharyngeal  pouches  of  a  10  mm.  pig 

embryo.     X  22.5. 


Neural  cavity 


Roots  of  n. 
hypoglossus 


Int.  jugular  vein 
N.n.  vagus  el 
accessorius 

Descend  Aorta 

rV.  facialis 
Br.  arch  Z 
Tongue 
Mandible 

Maxillary 
process 


is  the  external  branch  of  the  n.  accessorius,  and  mesial  to  the  ganglia  are  the  small  descending 
aorta:. 

Section  through  the  Fourth  Pharyngeal  Pouches  (Fig.  127). — This  region  is 
marked  by  the  disappearance  of  the  head  and  the  appearance  of  the  heart  in  the  pericardial 
cavity.  The  tips  of  the  atria  are  sectioned  as  they  project  on  either  side  of  the  bulbus  cordis. 
The  bulbus  is  divided  into  the  aorta  and  pulmonary  artery,  the  latter  connected  with  the  right 
ventricle,  which  has  spongy  muscular  walls.  The  pharynx  is  crescentic  and  continued  laterally 
as  the  small  fourth  pharyngeal  pouches.  Into  the  mid- ventral  wall  of  the  pharynx  opens  the 
vertical  slit  of  the  trachea.     A  section  of  the  vagus  complex  is  located  between  the  descending 


Op  ma  I  ganalion. 


Epiglottis 
Branch  arch  3 


Spinal  cord 


Intjuqular 
vein 

N.  hypogtossus 

Gang,  nodos. 
n.W 

Pharyngeal 
pouch  3 

Aortic  arch3 


Branch  arch 2 


MandibU 


Thyreoid  anlage, 


Olfactorypit 


Fig.  126. — Transverse  section  through  the  third  pharyngeal  pouches  of  a  10  mm.  pig  embryo.     X  22.5. 


Spinal,  qana. 

R.  desc.  aorta 

Pharynx 
Hvagus 

Tracheal  groove 
R.  atrium 
Aorta 
Rilmonary  artery 


Spinal  cord 


L.desc  aorta* 

Int.  jugular 
vein 

'Pharyngeal 
pouch  4 


L  atrium 


Pericardial 
cavity 


L.]/entrlc!e 


ft.venTricle 

Fig.  127. — Transverse  section  through  the  fourth  pharyngeal  pouches  of  a  10  mm.  pig  embryo.     X  22.5. 

136 


TRANSVKKSK    SECTIONS    OF    A    TEN    MM.    PIG    EMBRYO 


137 


aorta  and  the  internal  jugular  vein.  At  this  level  the  jugular  vein  receives  the  linguo-facial 
vein.  The  left  descending  aorta  is  larger  than  the  right.  The  ventral  aorta  may  be  traced 
cranially  in  the  scries  to  the  fourth  aortic  arches.  '1'hc  pulmonary  artery,  if  followed  caudad, 
connects  with  the  fifth  aortic  arches  as  in  Fig.  [28. 

Section   through    the  Fifth   Aortic    Arches    (Fig.    128).     The   fifth   aortic  arch   is 
complete  on  the  left  side.      From  these  pulmonary  arches  small  pulmonary  arteries  may  be  I 
caudad  in  the  series  to  the  lung  anlages.     The  cavity  of  the  pharynx  forms  a  curved  horizontal 
slit.      All  four  chambers  of  the  heart  are  represented,  but   the  aorta  and  pulmonary  artery  are 
incompletely  separated  by  the  right  and  left  bulbar  swellings  or  folds. 

Section  through  the   Sinus  Venosus  and  the  Heart  (Fig.    120).-  The  section  is 
marked  by  the  symmetrically  placed  atria  and  ventricles  of  the  heart  and  by  the  presence  of 


£pmal  qana 

Nohchord 

Kdesc  aorlc 
Esophagus 

Trachea 

Aorta 
/?.  atrium 


Cavity  of 
Bulbus 


fi.  ventricle 


Spinal  cord 


L  .Descending 
aorta 

Anf.  cardinal 
Vain 

N.  vagus 


L.  atnum 

Pulmonary 
artery 


L.ventrick 


Fig.  12S. — Transverse  section  through  the  fifth  pair  of  aortic  arches  and  bulbus  cordis  of  a  10  mm.  pig 

embryo.     X  22.$. 


the  upper  limb  buds.  Dorsal  to  the  atria  are  the  common  cardinal  veins,  the  right  vein  forming 
part  of  the  sinus  venosus.  The  sinus  venosus  drains  into  the  right  atrium  through  a  slitdike 
opening  in  the  dorsal  and  caudal  atrial  wall.  The  opening  is  guarded  by  the  right  and  left 
of  the  sinus  venosus.  which  project  into  the  atrium.  The  septum  primum  completely 
divides  the  right  and  left  atria  at  this  level,  which  is  caudal  to  the  foramen  ovale  and  the 
atria-ventricular  openings.  The  septum  joins  the  fused  endocardial  cushions.  Xote  that 
the  esophagus  and  trachea  are  now  tubular  and  that  the  left  descending  aorta  is  much  larger 
than  the  right.  Around  the  epithelium  of  both  trachea  anil  esophagus  are  condensations  of 
mesenchyma,  from  which  their  outer  layers  are  differentiated. 

Section  through  the  Foramen  Ovale  of  the  Heart  (Tig.  1^0). — The  level  of  this 
section  is  cranial  to  that  of  the  previous  figure  and  shows  the  septum  primum  interrupted  dor- 
sally  to  form  the  foramen  ovale.     Each  atrium  communicates  with  the  ventricle  of  the  same 


i*8 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER   PIG   EMBRYOS 


side  through  the  atrio-vcntricular  foramen.     Between  these  openings  is  the  endocardial  cushion, 
which  in  part  forms  the  anlages  of  the  tricuspid  and  bicuspid  valves.     The  atria  are  marked  off 


dpinal  gong. 
A/otocnord 

R.  c/esc.  Aorta. 
Sinus  venosus 

R.  valve sinus 
Venosus 

Pericardial  cavi/y 

R.  Ventricle 

Intervent  Septum 


tSj)'mal  cord 


Upper  limb  bud 
Esophagus 
L  com.  card,  vein 

Trachea 

L  Afr/'um 

Endocardial 
cushion 

Body  wall 


Fig.  129. — Transverse  section  through  the  sinus  venosus  of  the  heart  in  a  10  mm.  pig  embryo.     X  22.5. 
L.  com.  card,  vein,  left  common  cardinal  vein;   R.  desc.  Aorta,  right  descending  aorta. 


Foramen 
ovale 

R.AtriUm 


L.  Atrium 


Septum  1 

Endocardial 
cushion 

L.atrio-vent 
foramen 


L.Ventricle 


Intervent 
Septufr 


Fir,.  130. — Transverse  section  through  the  foramen  ovale  of  the  heart  in  a  10  mm.  pig  embryo. 
X  22.5.  />.  atrio-vcnt.  foramen,  R.  alrio-vent.  foramen,  left  and  right  atrio-ventricular  foramen; 
Intervent.  septum,  interventricular  septum. 


externally  from  the  ventricles  by  the  coronary  sulcus.     Between  the  two  ventricles  is  the  inter- 
ventricular septum.     The  ventricular  walls  are  thick  and  spongy,  forming  a  network  of  muscular 


TRANSVERSE    SECTIONS   OF   A   TEN   MM.    PIG   EMBRYO 


139 


cords  or  trabecular  surrounded  by  blood  spaces  or  sinusoids.  The  trabecule  are  composed  of 
muscle  cells,  which  later  become  striated  and  constitute  the  myocardium.  They  are  surrounded 
by  an  endothelial  layer,  the  endocardium.  From  the  blood  circulating  in  the  sinusoids  the 
mammalian  heart  receives  all  its  nourishment  until,  later,  the  coronary  vessels  of  the  heart  wall 
are  developed.  The  heart  is  surrounded  by  a  layer  of  mesothclium,  the  epicardium.  which 
is  continuous  with  the  pericardial  mesothelium  lining  the  body  wall. 

Section  through  the  Liver  and  Upper  Limb  Buds  (Fig.  131). — The  section  is 
marked  by  the  presence  of  the  upper  limb  buds,  the  liver  and  the  bifurcation  of  the  trachea  to 
form  the  primary  bronchi  of  the  lungs.  The  limb  buds  are  composed  of  dense  undifferentiated 
mesenchyme  surrounded  by  the  ectoderm  which  is  thickened  at  their  tips.     The  seventh  pair 


<5pinalqojiqlion 

Spinal  nerve 

Post,  cardinal 
vein 
Mesonephros 

Pleural  cavity 
Upper  limb  bud 


U 


ver 


Spinal  cord 


Noto  chord 


Descending 

dorta. 

Esophagus 

Bifurcation 
of  trachea 


Fig. 


131- 


-Transverse  section  through  the  liver  and  upper  limb  buds  of  a  10  mm.  pig  embryo  at  the  level 
of  the  bifurcation  of  the  trachea.     X  22.5.     Inf.  vena  cava,  inferior  vena  cava. 


of  cervical  ganglia  and  nerves  are  cut  lengthwise  showing  the  spindle-shaped  ganglia  with  the 
dorsal  root  fibers  taking  origin  from  their  cells.  The  ventral  root  fibers  arise  from  the  ventral 
cells  of  the  mantle  layer  and  join  the  dorsal  root  to  form  the  nerve  trunk.  On  the  right  side, 
a  short  dorsal  ramus  supplies  the  anlage  of  the  dorsal  muscle  mass.  The  much  larger  ventral 
ramus  unites  with  those  of  other  nerves  to  form  the  brachial  plexus. 

The  descending  aorta:  have  now  fused  and  the  seventh  pair  of  dorsal  intersegmental  arteries 
arise  from  the  dorsal  aorta.  From  these  intersegmental  arteries  the  subclavian  arteries  are  given 
off  two  sections  caudad  in  the  series.  Lateral  to  the  aorta  are  the  posterior  cardinal  veins. 
The  esophagus,  ventral  to  the  aorta,  shows  a  very  small  lumen,  while  that  of  the  trachea  is 
large  and  continued  into  the  bronchi  on  either  side.  The  lung  aidages  project  laterally  into 
the  crescentic  pleural  cavities,  of  which  the  left  is  separated  from  the  peritoneal  cavity  by  the 


140 


THE    STUDY    OF    SIX    AND    TEN    MILLIMETER   PIG    EMBRYOS 


septum  transversum.  The  liver,  with  its  fine  network  of  trabecule  and  sinusoids,  is  large  and 
nearly  fills  the  peritoneal  or  abdominal  cavity.  The  liver  cords  are  composed  of  liver  cells 
surrounded  by  the  endothelium  of  the  sinusoids.     Red  blood-cells  are  developed  in  the  liver 


Mfocnord 


Esoplmy, 

Lesser  peri- 
toneal sac' 

Inf.  vena  cava 


Qanq. 
Post  card,  vein 


Mesonephrt'c 
tubule 

Peritoneal 
cavity 

D.hbe  of 
Liver- 


Sinusoids  of 
liver 


Ductus  venosus/ 

Fig.  132. — Dorsal  half  of  a  transverse  section  through  the  lung  buds  cranial  to  the  stomach  in  a 
10  mm.  pig  embryo.     X  22.5.     Post.  card,  vein,  posterior  cardinal  vein. 


Spinal  cord 
Notocnord 
Dorsal  aorta 

Plica  venae 
cai/ae 

Inf.  Vena  cava 
lesser  omentum 


Spinal  gano. 


Base  of.    , 
upper  limb 


Glomerulus  of 
Tneso  nephros 

Greater  omentum 
Stomach 
D.  lobe  of  liver 
Ductus  venosus 

l/.lobe  of  liver 


Ventral  attachment, 
of  Liver 


FlG.  i  5.3. — Transverse  section  through  the  stomach  and  liver  of  a  10  mm.  pig  embryo.     X  22.5. 


TRAXSVKRSK    SKl  TK>\S    ()F    A    TK.Y    MM.    PIG    EMBRYO 


141 


at  this  stage.  The  large  vein  penetrating  the  septum  transversum  from  the  liver  to  the  heart. 
is  the  proximal  portion  of  the  inferior  vena  cava,  originally  the  right  vitelline  vein.  Ventral 
to  the  bronchi  may  Ik-  seen  sections  of  the  pulmonary  veins. 

Section  through  Lung  Buds  Cranial  to  Stomach  I  Fig.  132). — The  lungs  are 
sectioned  through  their  caudal  ends  ami  the  esophagus  is  just  beginning  to  dilate  into  the 
stomach.  On  either  side  of  the  circular  dorsal  aorta  are  the  mesoncphroi.  The  pleural  cavities 
now  communicate  freely  on  both  sides  with  the  peritoneal  cavity.  A  section  of  the 
peritoneal  sac  appears  as  a  crescent-shaped  slit  to  the  right  of  the  esophagus.  In  the  right 
dorsal  lobe  of  the  liver  is  located  the  inferior  vena  cava.  Near  the  median  line  ventral  to  the 
lesser  sac  is  the  large  ductus  venosus. 


Spinal  cord 


Notochord 
Mesoneph 


Plica  venae 
cavae 

Inf.  vena  cava 

Lesser  peritoneal 
Sac 

Portal  vein 

Hepatic 
diverticulum 


ft.  Umbilical 
vein 


Sympathetic 
ramus 

Dorsal  aorta 


Dorsal 
mesoaastnum 

Mesonephr'ic 
duct 

Stomach 


Ventral  lobe 
Liver 


L.Urnbi  lical 
Vein 


Fig.  134. — Transverse  section  through  the  hepatic  diverticulum  of  a   10  mm.  pig  embryo.     X  22.5. 


Section  through  the  Stomach  and  Liver  (Fig.  133 ). — Prominent  in  the  body 
cavity  are  the  mesoncphroi  and  liver  lobes.  The  mesonephroi  show  sections  of  coiled  tubules 
lined  with  cuboidal  epithelium.  The  glomeruli,  or  renal  corpuscles,  are  median  in  position 
and  develop  as  knots  of  small  arteries  which  grow  into  the  ends  of  the  tubules.  The  thickened 
epithelium  along  the  median  and  ventral  surface  of  the  mesonephros  is  the  anlage  of  the  genital 
gland.  The  body  wall  is  thin  and  lined  with  mesothelium  continuous  with  that  which  covers 
the  mesenteries  and  organs.  The  mesothelial  layer  becomes  the  epithelium  of  the  adult  peri- 
toneum, mesenteries  and  serous  layer  of  the  viscera.  The  stomach  lies  on  the  left  side  and  is 
attached  dorsally  by  the  greater  omentum,  ventrally  to  the  liver  by  the  lesser  omentum.  The 
right  dorsal  lobe  of  the  liver  is  attached  dorsally  to  the  right  of  the  great  omentum.  In  the  liver 
ventral  to  this  attachment  courses  the  inferior  vena  cava  and  the  attachment  forms  the  plica 
voice  cavec.     Between  the  attachments  of  the  stomach  and  liver,  and  to  the  right  of  the  stomach, 


142 


THE    STUDY    OF    SIX   AND    TEN   MILLIMETER  PIG  EMBRYOS 


is  the  lesser  peritoneal  sac.  In  the  liver  to  the  left  of  the  midline  is  the  ductus  venosus,  sectioned 
just  at  the  point  where  it  receives  the  left  umbilical  vein  and  a  branch  from  the  portal  vein. 
The  ventral  attachment  of  the  liver  later  becomes  the  falciform  ligament. 

Section  through  the  Hepatic  Diverticulum  (Fig.  134). — The  section  passes  through 
the  pyloric  end  of  the  stomach  and  the  duodenum  near  the  attachment  of  the  hepatic  diverti- 
culum. The  great  omentum  of  the  stomach  is  larger  than  in  the  previous  section  and  to  its 
right,  in  the  plica  venae  cavae,  lies  the  inferior  vena  cava.  Ventral  to  the  inferior  vena  cava 
is  a  section  of  the  portal  vein.  The  ventral  and  dorsal  lobes  of  the  liver  are  now  separate  and 
in  the  right  ventral  lobe  is  embedded  the  saccular  end  of  the  hepatic  diverticulum,  which  forms 
the  gall  bladder.  To  the  right  of  the  stomach,  the  diverticulum  is  sectioned  again  just  as  it 
enters  the  duodenum.  Ventrally  the  left  umbilical  vein  is  entering  the  left  ventral  lobe  of  the 
liver.  It  is  much  larger  than  the  right  vein,  which  still  courses  in  the  body  wall.  On  the 
left  side  of  the  embryo  the  spinal  nerve  shows  in  addition  to  its  dorsal  and  ventral  rami  a  sympa- 
thetic ramus,  the  fibers  of  which  pass  to  a  cluster  of  ganglion  cells  located  dorso-lateral  to  the 


Dorsal  Aorta 


Inf.  Vena  cava 


R.  vitelline  or 
portal  vein 

Mesonephric 
duct 


Post  cardinal 
l/ein 


Mesonephros 


L. Vitelline 
I   vein 

Dorsal  pancreas 
Liver 


Duodenum 
\Jentral  pancreas- 

Fig.  135. — Portion  of  a  transverse  section  through  the  pancreatic  anlages  of  a  10  mm.  pig  embryo. 

X  22.5. 


aorta.  These  cells  form  one  of  a  pair  of  sympathetic  ganglia  and  are  derived  from  a  spinal 
ganglion. 

Section  through  the  Pancreatic  Anlages  (Tig.  135). — The  lesser  peritoneal  sac  just 
above  the  level  of  this  section  has  opened  into  the  peritoneal  cavity  through  the  epiploic 
foramen  (of  Winslow).  The  mesonephric  ducts  are  now  prominent  ventrally  in  the  mesonephroi. 
The  duct  of  the  dorsal  pancreas  is  sectioned  tangentially  at  the  point  where  it  takes  origin  from 
the  duodenum.  From  the  duct  the  lobulated  gland  may  be  traced  dorsad  in  the  mesentery. 
To  the  right  of  the  dorsal  pancreatic  duct  is  a  section  of  the  ventral  pancreas,  which  may  be 
traced  cephalad  in  the  series  to  its  origin  from  the  hepatic  diverticulum.  Dorsal  to  the  ventral 
pancreas  is  a  section  of  the  portal  vein.  The  inferior  vena  cava  appears  as  a  vertical  slit  in 
the  dorsal  mesentery. 

Section  through  the  Urogenital  Sinus  and  the  Lower  Limb  Buds  (Fig.  136). — 
The  figure  shows  only  the  caudal  end  of  a  section,  in  the  dorsal  portion  of  which  the  mesonephroi 
were  sectioned  at  the  level  of  the  subcardinal  anastomosis.  A  portion  of  the  mesentery  is  shown 
with  a  section  of  the  colon.  In  the  body  wall  are  veins  which  drain  into  the  umbilical  veins, 
and  on  each  side  are  the  umbilical  arteries,  just  entering  the  body  from  the  umbilical  cord.  Be- 
tween them,  in  sections  cranial  to  this,  the  allantoic  stalk  is  located.     Here  it  has  opened 


TRANSVERSE    SECTIONS    OF   A   TEN   MM.    PIG   EMBRYO 


143 


into  the  crescent ic  urogenital  sinus.  Dorsal  to  the  urogenital  sinus  (dorsal  now  being  at  the 
bottom  of  the  figure  owing  to  the  curvature  of  the  caudal  region)  is  a  section  of  the  rectum, 
separated  from  the  sinus  by  a  curved  prolongation  of  the  ccelom.     From  the  ends  of  the  uro- 


Muenterf 


CaudaJ  limh. 
of  Intestine 

Lower 
limb  bud 


Mesonephric 
duct 


Notochord 


Vein  in  body  wall 
L. umbilical  artery 

Allantoit  and  uro- 
genital sinus 

Rectum 


Spinal  cord 


Fig.  136. — Transverse  section  through  the  urogenital  sinus  and  rectum  of  a  10  mm.  pig  embryo.     X  22.5. 


Lowerlimb 
bud 

P.umb'ilicaJ 
artery 


Notochord 


Caudal  limb 
of  Intestine 


Fig.  137. — Transverse  section  of  a  10  mm.  embryo  passing  through  the  lower  limb  buds  at  the  level  of 
the  openings  of  the  ureters  into  the  mesonephric  ducts.     X  22.5. 


*&&£M  t^feT>^@K<g2 Cotton 

Mesentery  —^^^mw^\0£:'  ■ 

1'%&^&W-2K'    If: 

X)     .m    i     Li 

$m     ^w!  ^     -  '-'~-:^y  Metanephros 

j  ^     / 
LW«  -iO _Notochord 


R.umbihcal 
artery 

Vein 


Spinal  cord 
Fig.  138. — Transverse  section  through  the  anlages  of  the  metanephroi  in  a  10  mm.  pig  embryo.     X  22.5. 


144  THE    STUDY    OF    SIX   AND    TEN    MILLIMETER    PIG    EMBRYOS 

genital  sinus,  as  we  trace  cephalad  in  the  embryo  {downward  in  the  series),  are  given  off  the 
mesonephric  ducts. 

Section  through  the  Mesonephric  Ducts  at  the  Opening  of  the  Ureter  (Fig.  137). 
— The  section  cuts  near  the  middle  through  both  lower  limb  buds.  Mesial  to  their  bases 
are  the  umbilical  arteries,  which  He  lateral  to  the  mesonephric  ducts.  From  the  dorsal  wall  of 
the  left  mesonephric  duct  is  given  off  the  ureter  or  duct  of  the  melanephros.  Tracing  the  sections 
down  in  the  series,  both  ureters  appear  as  minute  tubes  in  transverse  section.  They  soon 
dilate  to  form  the  pelvis  of  the  kidney  at  the  level  of  Fig.  138.  Note  the  undifferentiated  mesen- 
chyme of  the  lower  limb  buds  and  their  thickened  ectodermal  tips. 

Section  through  the  Metanephroi  and  Umbilical  Arteries  (Fig.  138). — The  sec- 
tion passes  caudal  to  the  mesonephric  ducts  which  curve  along  the  ventral  surfaces  of  the 
mesonephroi  (Fig.  119).  The  umbilical  arteries  course  lateral  to  the  metanephroi  which  con- 
sist merely  of  the  thickened  epithelium  of  the  pelvis  surrounded  by  a  layer  of  condensed  mesen- 
chyma,  the  nephrogenic  tissue. 


CHAPTER  VI 

THE  DISSECTION  OF  PIG  EMBRYOS  FOR  STUDY:  DEVELOPMENT  OF 
FACE,  PALATE,  TONGUE,  SALIVARY  GLANDS  AND  TEETH 

As  the  average  student  will  not  have  time  to  study  series  of  embryos  sec- 
tioned in  different  planes,  dissections  may  be  used  for  showing  the  form  and  re- 
lations of  the  organs.  Cleared  embryos  mounted  whole  are  instructive,  but 
show  the  structures  superimposed  and  are  apt  to  confuse  the  student.  Pig  em- 
bryos 10  mm.  or  more  in  length  may  be  easily  dissected,  mounted  as  opaque 
objects  and  used  for  several  years.  Success  in  dissecting  such  small  embryos 
depends  (i)  on  the  fixation  and  hardening  of  the  material  employed;  (2)  on  start- 
ing the  dissection  with  a  clean  cut  in  the  right  plane;  (3)  on  a  knowledge  of  the 
anatomy  of  the  parts  to  be  dissected. 

Fixation  and  Hardening  of  Material. — Embryos  fixed  in  Zenker's  fluid  have 
given  the  best  results.  They  should  then  be  so  hardened  in  95  per  cent,  alcohol 
that  the  more  diffuse  mesenchyma  will  readily  separate  from  the  surfaces  of  the 
various  organs,  yet  the  organs  must  not  be  so  brittle  that  they  will  crumble  and 
break.  Embryos  well  hardened  and  then  kept  for  two  weeks  in  80  per  cent, 
alcohol  usually  dissect  well.  Old  material  is  usually  too  brittle,  that  just  fixed 
and  hardened  may  prove  too  soft.  As  a  test,  determine  whether  the  mesenchyma 
separates  readily  from  the  cervical  ganglia  and  their  roots. 

Dissecting  Instruments  include  a  binocular  dissecting  microscope,  a  sharp 
safety  razor  blade,  large  curved  blunt-pointed  dissecting  needles,  pairs  of  small 
sharp-pointed  forceps  and  straight  dissecting  needles  small  and  large. 

Methods  of  Dissection. — In  general,  it  is  best  to  begin  the  dissection  with 
a  clean,  smooth  cut  made  by  a  single  stroke  with  the  safety  razor  blade,  which 
should  be  flooded  with  80  per  cent,  alcohol.  The  section  is  made  free  hand 
holding  the  embryo,  protected  by  a  fold  of  absorbent  cotton,  between  the  thumb 
and  index  finger.  Having  made  preliminary  cuts  in  this  way,  the  embryo  may 
be  affixed  with  thin  celloidin  to  a  cover  glass  and  immersed  in  a  watch-glass  con- 
taining alcohol.  We  prefer  not  to  affix  the  embryo,  as  the  celloidin  used  for  this 
purpose  may  interfere  with  the  dissection.  Instead,  a  cut  is  made  parallel  to 
the  plane  of  the  dissection  so  that  the  embryo,  resting  in  the  watch-glass  upon 
10  145 


146  THE  DISSECTION   OF   PIG  EMBRYOS   FOR   STUDY 

this  flat  surface,  will  be  in  a  fairly  stable  position.  It  may  thus  be  held  in  any 
convenient  position  by  resting  the  convex  surface  of  a  curved  blunt  dissecting 
needle  upon  some  part  not  easily  injured.  The  dissection  is  then  carried  on  under 
the  binocular  microscope,  using  the  fine  pointed  forceps,  dissecting  needles,  and 
a  small  pipette  to  wash  away  fragments  of  tissue. 

Whole  Embryos. — For  the  study  of  the  exterior,  whole  embryos  may  be 
affixed  with  celloidin  to  the  bottoms  of  watch-glasses  which  may  be  stacked  in 
wide-mouthed  jars  of  80  per  cent,  alcohol.  The  specimens  may  thus  be  used 
several  years  at  a  saving  of  both  time  and  material.  Preliminary  treatment  con- 
sists in  immersion  in  95  per  cent,  alcohol  one  hour,  in  ether  and  absolute  alcohol 
at  least  thirty  minutes,  in  thin  celloidin  one  hour  or  more.  Pour  enough  thin 
celloidin  into  a  Syracuse  watch-glass  to  cover  its  bottom,  and  immerse  in  this  a 
circle  of  black  mat  paper,  first  wet  with  ether  and  absolute  alcohol.  Pour  off 
any  surplus  celloidin,  mount  embryo  in  desired  position  and  immerse  watch-glass 
in  80  per  cent,  alcohol,  in  which  the  specimen  may  be  kept  indefinitely.  Embryos 
may  also  be  mounted  in  gelatine-formalin  solution  in  small  sealed  glass  jars. 

Lateral  Dissections  of  the  Viscera. — Dissections  like  those  shown  in  Figs. 
139  and  140  may  easily  be  prepared  in  less  than  an  hour,  and  make  valuable 
demonstration  and  laboratory  specimens.  Skill  is  required  to  demonstrate  most 
of  the  cerebral  nerves,  but  the  central  nervous  system,  cerebral  and  spinal  ganglia 
and  viscera  may  easily  be  exposed.  Starting  dorsally,  make  a  sagittal  section 
of  the  embryo  slightly  to  one  side  of  the  median  line  and  avoiding  the  umbilical 
cord  ventrally.  With  the  embryo  resting  on  the  flat  sectioned  surface,  begin 
at  the  cervical  flexure  and  with  fine  forceps  grasp  the  ectoderm  and  dural  anlage 
at  its  cut  edge,  separate  it  from  the  neural  tube  and  pia  mater  and  strip  it  off 
ventralwards  exposing  the  myelencephalon  and  cervical  portion  of  the  cord. 
As  the  mesenchyma  is  pulled  away,  the  ganglia  and  roots  of  the  cerebral  nerves 
will  be  exposed.  The  mesenchyma  between  the  ganglia  and  along  the  nerves 
may  be  removed  with  the  end  of  a  small  blunt  needle.  Care  must  be  exercised 
in  working  over  the  mesencephalon  and  telencephalon  of  the  brain  not  to  injure 
the  brain  wall,  which  may  be  brittle.  By  starting  with  a  clean  dissection  dor- 
sally  and  gradually  working  ventrad,  the  more  important  organs  may  be  laid 
bare  without  injury.  The  beginner  should  compare  his  specimen  with  the  dis- 
sections figured  and  also  previously  study  the  reconstructions  of  Thyng  (191 1) 
and  Lewis  (1902). 

Lateral  dissections  of  embryos  18  mm.  and  35  mm.  long  show  infinitely  better 
than  sections  the  form  and  relations  of  the  organs,  their  relative  growth  and 


LATERAL   DISSECTIONS    OF   THE   VISCERA 


147 


their  change  of  position  (Figs.  139  and  140).  Compare  the  organs  of  6,  10,  18 
and  35  mm.  embryos  and  note  the  rapid  growth  of  the  viscera  (see  Figs.  90  and 
115).  Hand-in-hand  with  the  increased  size  of  the  viscera  goes  the  diminution 
of  the  dorsal  and  cervical  flexures.     In  the  brain,  note  the  increased  size  of  the 


Mesencephalon        N.  octdomotorius 


Cerebellum 
Gang,  genieitlatum  n.  7 

Gang,  acust.  n.  S 

CitUlg.  SUp.  II.  Q 

Cuing,  accessor. 


N.  trochlcaris 

Gang,  scinilun.  n.  5 


Gattg.  jugular c  n.  10 

Gang,  petrosal  n.  9 
.V.  hypoglossus 

N .  aeeessorius 
Gang,  cerv 

Brachial  plcxus^rrW 


Lung-^9 

1 

Diaphragm  — 
Dorsal  lobe  liver 

Mesoncphros 

Metanephros 
Nerve  to  lower  li 


Mandibular  ramus  n.  5 

Ophthalmic  ramus  n.  5 
N.  opticus 

Cerebrum 

Maxillary  ramus  n.  5 
Chord. lyntp.  n.  7 
N.  facialis 

Gang,  uodos.  n.  10 

R.  atrium 
R.  ventricle 

Ventral  lobe  liver 
Umbilical  cord 


Sciatic  nerve 


Fig.  139.— Lateral  dissection  of  an  18  mm.  pig  embryo,  showing  the  nervous  system  and  viscera  from 

the  right  side.     X  15. 

cerebral  hemispheres  of  the  telencephalon  and  presence  of  the  olfactory  lobe  of 
the  rhinencephalon.  The  cerebellum  also  becomes  prominent  and  a  ventral 
flexure  in  the  region  of  the  pons,  the  pontine  flexure,  is  more  marked.  The  brain 
grows  relatively  faster  than  the  spinal  cord  and,  by  the  elongation  of  their  dorsal 


148 


THE   DISSECTION   OF   PIG  EMBRYOS   TOR   STUDY 


roots,  the  spinal  ganglia  are  carried  ventral  to  the  cord.  The  body  of  the  embryo 
also  grows  faster  than  the  spinal  cord,  so  that  the  spinal  nerves  at  first  directed 
at  right  angles  to  the  cord  course  obliquely  caudad  in  the  lumbo-sacral  region. 


Semilunar  ganglion  n.  5      Ophthalmic  ramus  n.  5 
Geniculate  gang.  n.  7       |  /      Cerebrum 


Mesencephalon 
Cerebellum         \a 


Gang.  n.  8 


Hypophysis 

N.  opticus 

Lobus  olfaclorius 


Gang.  sup.  n.  9 

G.  ju gul are  n.  10 

G.  Froriep 

Auricular  r.  n.  10 

Gang.  n.  cerv.  1 

Gang,  petros.  n.  9 

N.  accessorius 

N.  hypoglossus 

Gang.  cerv.  5-8 

Gang.  thor.  1 


Lung'fs, 

Diaphragm 

Dorsal  lobe  liver 

Mesonephros 

Metanephros 

Lumbar  gang 


'Maxillary  ramus  n.  5 
Mand.  ramus  n.  5 
Chorda,  tymp.  n.  7 

N.  facialis  (7) 
Gang,  nodosum  n.  10 


Nerve  to  lower  limb 


R.  atrium 
R.  ventricle 

Ventral  lobe  liver 
Umbil.  cord 
Lower  limb 


Sciatic  nerve 


FlG.  140. — Lateral  dissection  of  a  35  mm.  pig  embryo  to  show  the  nervous  system  and  viscera  from  the 

right  side.     X  llA- 

Median  Sagittal  Dissections  (Figs.  141  and  142). — Preliminary  to  the 
dissection,  a  cut  is  made  dorsally  as  near  as  possible  to  the  median  sagittal  plane. 
Beginning  caudally  at  the  mid-dorsal  line  an  incision  is  started  which  extends  in 


MEDIAN   SAGITTAL   DISSECTIONS  1 49 

depth  through  the  neural  tube  and  the  anlages  of  the  vertebrae.  This  incision  is 
carried  to  the  cervical  flexure,  cranial  to  which  point  the  head  and  brain  are 
halved  as  accurately  as  possible.  The  blade  is  then  carried  ventrally  and  cau- 
dallv,  cutting  through  the  heart  and  liver  to  the  rigid  of  the  midline  and  of  the 
umbilical  cord  until  the  starting  point  is  reached.  A  parasagittal  section  is  next 
made  well  to  the  left  of  the  median  sagittal  plane  and  the  sectioned  portion  is 
removed,  leaving  on  the  left  side  of  the  embryo  a  plane  surface.  With  the  embryo 
resting  upon  this  flat  surface,  the  dissection  is  begun  by  removing  with  forceps 
the  right  half  of  the  head.  In  pulling  this  away  caudalwards,  half  of  the  dorsal 
body  wall,  the  whole  of  the  lateral  body  wall,  and  the  parts  of  the  heart  and  liver 
lying  to  the  right  of  the  midline  will  be  removed,  leaving  the  other  structures  in- 
tact. If  the  plane  of  section  was  accurate,  the  brain  and  spinal  cord  will  be 
halved  in  the  median  sagittal  plane.  Wash  out  the  cavities  of  the  brain  with  a 
pipette  and  its  internal  structure  may  be  seen.  Dissect  away  the  mesenchyma 
between  the  esophagus  and  trachea  and  expose  the  lung.  Remove  the  right 
mesonephros,  leaving  the  proximal  part  of  its  duct  attached  to  the  urogenital 
sinus.  The  right  dorsal  lobe  of  the  liver  will  overlie  the  stomach  and  pancreas. 
Pick  it  away  with  forceps  and  expose  these  organs.  Dissect  away  the  caudal 
portion  of  the  liver  until  the  hepatic  diverticulum  is  laid  bare.  It  is  whitish  in 
color  and  may  thus  be  distinguished  from  the  brownish  liver.  Beginning  at  the 
base  of  the  umbilical  cord,  carefully  pull  away  its  right  wall  with  forceps,  thus 
exposing  the  intestinal  loop  and  its  attachment  to  the  yolk  stalk.  If  in  the  caudal 
portion  of  the  umbilical  cord  the  umbilical  artery  is  removed,  the  allantoic  stalk 
may  be  dissected  out.  To  see  the  anlage  of  the  genital  gland,  break  through  and 
remove  a  part  of  the  mesentery,  exposing  the  mesial  surface  of  the  left  meso- 
nephros and  the  genital  fold.  The  dissection  of  the  metanephros  and  ureter  is 
difficult  in  small  embryos.  In  10  to  12  mm.  embryos,  the  umbilical  artery,  just 
after  it  leaves  the  aorta,  passes  lateral  to  the  metanephros  and  thus  locates  it. 
By  working  carefully  with  tine  needles  the  surface  of  the  metanephros  may  be 
laid  bare  and  the  delicate  ureter  may  be  traced  to  the  base  of  the  mesonephric 
duct.  The  extent  of  the  dorsal  aorta  may  also  be  seen  by  removing  the  surround- 
ing mesenchyma.  With  a  few  trials,  such  dissections  may  be  made  in  a  short 
time,  and  are  invaluable  in  giving  one  an  idea  of  the  form,  positions  and  rela- 
tions of  the  different  organs.  By  comparing  the  early  (Figs.  91  and  117)  with 
the  later  stages  (Figs.  141  and  142)  a  number  of  interesting  points  may  be  noted: 
In  the  brain,  the  corpus  striatum  develops  in  the  floor  of  the  cerebral  hemi- 
spheres.    The  interventricular  foramen  is  narrowed  to  a  slit.     In  the  roof  of  the 


I^O 


THE    DISSECTION   OF   PIG  EMBRYOS   FOR   STUDY 


diencephalon  appears  the  anlage  of  the  epiphysis  or  pineal  gland,  and  the  chorioid 
plexus  of  the  third  ventricle.  This  extends  into  the  lateral  ventricles  as  the 
lateral  chorioid  plexus.     The  dorso-lateral  wall  of  the  diencephalon  thickens  to 


Isthmus         Mesencephalon 


Metencephalon 
Chorioid  plexus  fourth  ventricle 
Hypophysis 

Tela  chorioidca  fourth  ventricle 
Myelencephalon, 

Epiglottis 

Xotochord 

Esophagus 

Trunk  pulm.  artery 

Wall  of  atrium 

Foramen  ovale 

Trachea 

Lung 

Dorsal  aorta 

Stomach 

Intersegmental 
arteries 

Pancreas 

Common  bile  duel 

Duodenum 

Genital  ridge 

Metanephr'os 


Mesonephric  duct  Ureter 


Third  ventricle 

Diencephalon 

Corpus  striatum 

Cerebrum 

Tongue 
Aorta 

Semilunar  valves 

R.  ventricle 

Yolk-sac 

Diaphragm 

Yolk-stalk 

Liver 

Ccecutn 

Colon 

Small  intestine 

Allanlois 

Bladder 

Phallus 

Urogenital  sinus 

Rectum 


Fig.  141.— Median  sagittal  dissection  of  an  18  mm.  pig  embryo,  showing  central  nervous  system  in 

section  and  the  viscera  in  position. 

form  the  thalamus  and  the  third  ventricle  is  narrowed  to  a  vertical  slit.  The 
increased  size  of  the  cerebellum  has  been  noted.  Into  the  thin  dorsal  wall  of  the 
myelencephalon  grows  the  network  of  vessels  which  form  the  chorioid  plexus  of 


MEDIAN    SAGITTAL   DISSECTIONS 


I>I 


the  fourth  ventricle,  which  is  now  spread  out  laterally  and  flattened  dorso-ven- 

trallv.     About  the  notochord  mesenchymal  anlages  which  form  the  centra  of  the 
vertebra  are  prominent. 

Turning  to  the  alimentary  tract,  observe  that  the  primitive  mouth  cavity 


Pedunc.  cerebri 

Cerebellum 
Chorioidal  plexus  veti'-> 


Tela  of  ventricle  4 
Myelencephalon 

Epiglottis 

Esophagus  - 
Spinal  cord 

Trachea 

A  orta 
R.  atrium 
R.  bronchus 
Dorsal  acrta 
>ia  cava 
Stomach 
Pancreas 
Suprarenal  gland 
Genital  gland 

Duodenum —      \  \ 

Metanephros 

Colon 
L.  mesonephri, 

I  'reter  / 

Urogenital  sinus  with  mesonephric  duct 


Epiphysis       Thalamus 
M  ■  it ncephalon 

Tela  chorioidea 


Lai.  chorioid  plexus 

Corpus  striatum 

Hypophysis 
Lobus  ol/actorius 
Turbinate  anlage 

Palate 


Pulmonarx  arlerv 


Gall  bladder 
Small  intestine 


Rectum 


Fig.   142. — Median  sagittal  dissection  of  a  35  mm.  embryo. 

is  now  divided  by  the  palatine  folds  into  the  upper  nasal  passages  and  lower 
oral  cavity.  In  the  lateral  walls  of  the  nasal  passages  develop  the  anlages  of  the 
turbinate  bones.  On  the  floor  of  the  mouth  and  pharynx,  the  tongue  and  epiglottis 
become  more  prominent.     The  trachea  and  esophagus  elongate  and  the  lungs  He 


152 


THE   DISSECTION   OF   PIG  EMBRYOS   FOR   STUDY 


Anla-qe . 

Mulleria., 

duct 


more  and  more  caudad.  The  dorsal  portion  of  the  septum  transversum,  the 
anlage  of  a  portion  of  the  diaphragm,  is  thus  carried  caudad  and  although  origi- 
nally, when  traced  from  the  dorsal  body  wall,  it  was  directed  caudad  and  ventrad 
now  it  curves  cephalad  and  ventrad,  bulging  cephalad  into  the  thorax.  The 
proximal  limb  of  the  intestinal  loop  elongates  rapidly  and,  beginning  with  the 
duodenum,  becomes  flexed  and  coiled  in  a  characteristic  manner.  The  distal 
limb  of  the  intestinal  loop  is  not  coiled,  but  its  diverticulum,  the  ccecum,  is  more 
marked.  Caudally  the  rectum,  or  straight  gut,  has  completely  separated  from 
the  urogenital  sinus  and  opens  to  the  exterior  through  the  anus. 

Of  the  urogenital  organs,  the  genital  folds  have  become  the  prominent  genital 

glands  attached  to  the  med- 
ian surfaces  of  the  meso- 
nephroi.  The  metanephroi 
have  increased  rapidly  in 
size  and  have  shifted  ceph- 
alad. The  proximal  portion 
of  the  allantoic  stalk  has  di- 
lated and,  with  the  adjacent 
part  of  the  urogenital  sinus, 
forms  the  bladder.  As  the 
urogenital  sinus  grows  it 
takes  up  into  its  wall  the 
proximal  ends  of  the  meso- 
nephric  ducts,  so  that  these 
and  the  ureters  have  sepa- 
rate openings  into  the  sinus. 
Owing  to  the  unequal  growth 
of  the  sinus  wall,  the  ureters  cpen  near  the  base  of  the  bladder,  the  mesonephric 
ducts  more  caudally  into  the  urethra.  The  phallus  now  forms  the  penis  of  the 
male  or  the  clitoris  of  the  female.  Cranial  to  the  metanephros  a  new  organ, 
the  suprarenal  gland,  has  developed.  These  are  ductless  glands  and  are  much 
larger  in  human  embryos. 

The  heart,  as  may  be  seen  by  comparing  Figs.  91  and  142,  although  at  first 
pressed  against  the  tip  of  the  head,  shifts  caudally  until  in  the  35  mm.  embryo  it 
lies  in  the  thorax  opposite  the  first  five  thoracic  nerves.  Later  it  shifts  even 
further  caudad.  The  same  is  true  of  the  other  internal  organs,  the  metanephros 
excepted.     As  the  chief  blood-vessels  are  connected  with  the  heart  and  viscera, 


/ll/anfoi 


CoeLom 


Fig.  143. — Ventral  dissection  of  a  15  mm.  embryo,  show- 
ing lungs,  digestive  canal  and  mesonephroi.  The  ventral  body 
wall,  heart  and  liver  have  been  removed  and  the  limb  buds 
cut  across.     X  6. 


DEVEI.oIWIEN  I     of     I  HE    FACE 


153 


profound  changes  in  the  positions  of  the  vessels  are  thus  brought  about,  for  the 
vessels  must  shift  their  positions  with  the  organs  which  they  supply. 

Ventral  Dissections. — Ventral  dissections  of  the  viscera  are  very  easily 
made.  With  the  safety  razor  blade,  start  a  cut  in  a  coronal  plane  through 
the  caudal  end  of  the  embryo  and  the  lower  limb  buds  (Fig.  143).  Extend  this 
cut  laterad  and  cephalad  through  the  body  wall  and  the  upper  limb  bud.  The 
head  may  be  cut  away  in  the  same  plane  of  section  and  the  cut  continued  through 
the  body  wall  and  upper  limb  bud  of  the  opposite  side  back  caudally  to  the  start- 
ing point.  Section  the  embryo  in  a  coronal  plane,  parallel  with  the  first  section 
and  near  the  back,  so  that  the  embryo  will  rest  upon  the  flattened  surface.  With 
forceps,  now  remove  the  ventral  body  wall.  By  tearing  open  the  wall  of  the 
umbilical  cord  along  one  side  it  may  be  removed,  leaving  the  intestinal  loop  in- 
tact. Pull  away  the  heart,  noting  its  external  structure.  The  liver  may  also 
be  removed,  leaving  the  stomach  and  intestine  uninjured.  A  portion  of  the 
septum  transversum  covering  the  lungs  may  be  carefully  stripped  away  and  the 
lungs  thus  laid  bare.  Dissections  made  in  this  way  show  the  trachea  and  lungs, 
the  esophagus,  stomach  and  dorsal  attachment  of  the  septum  transversum,  the 
course  of  the  intestinal  canal,  and  also  the  mesonephroi  and  their  ducts.  Favor- 
able sections  through  the  caudal  end  of  the  body  may  show  the  urogenital  sinus, 
rectum  and  sections  of  the  umbilical  arteries  and  allantois  (Figs.  92,  119  and  143). 
In  late  stages,  by  removing  the  digestive  organs  the  urogenital  ducts  and  glands 
are  beautifully  demonstrated  (Figs.  216  and  217). 


DEVELOPMENT  OF  THE  FACE 

The  heads  of  pig  embryos  have  long  been  used  for  the  study  of  the  devel- 
opment of  the  face.  The  heads  should  be  removed  by  passing  the  razor  blade 
between  the  heart  and  adjacent  surface  of  the  head,  severing  the  neck.  Xext 
cut  away  the  dorsal  part  of  the  head  by  a  section  parallel  to  the  ventral  surface, 
the  razor  blade  passing  dorsal  to  the  branchial  clefts  and  eyes.  Mount  ventral 
side  up  three  stages  from  embryos  6,  12  and  14  mm.  long  as  shown  in  Figs. 
92  and  144  A  and  B. 

In  the  early  stages  (Figs.  92  and  119)  the  four  branchial  arches  and  clefts 
are  seen.  The  third  and  fourth  arches  soon  sink  into  the  cervical  sinus,  while 
the  mandibular  processes  of  the  first  arch  are  fused  early  to  form  the  lower  jaw. 
The  frontal  process  of  the  head  is  early  divided  into  lateral  and  median  nasal 
processes  by  the  development  of  the  olfactory  pits.     The  processes  are  distinct 


154 


THE   DISSECTION   OF   PIG   EMBRYOS   FOR   STUDY 


and  most  prominent  at  12  mm.  (Fig.  144  A).  Soon,  in  13  to  14  mm.  embryos, 
the  median  nasal  processes  fuse  with  the  maxillary  processes  of  the  first  arch  and 
constitute  the  upper  jaw  (Fig.  144  B).  The  lateral  nasal  processes  fuse  with  the 
maxillary  processes  and  form  the  cheeks,  the  lateral  part  of  the  lips  and  the  alae 
of  the  nose.  Later,  the  median  nasal  processes  unite  and  become  the  median 
part  of  the  upper  lip  and  the  columna  nasi. 

The  early  development  of  the  face  is  practically  the  same  in  human  embryos 


Lat.  nasal  process 

Olfactory  pit 

Med.  nasal  process 

Mandible 

Br.  arch  II 
Ventral  aorta 


Eye 

Lacrymal  groove 

Maxillary  proc. 

Br.  cleft.  I 
Br.  cleft  II 


Lat.  nasal  process 

Maxillary  process 
Mandible 
Br.  cleft  I 


Ext.  naris 

Eye 

Med.  nasal  process 

Oral  cavity 

Ext.  ear 


Fig.  144. — Two  stages  showing  the  development  of  the  face  in  pig  embryos.     A,  Ventral  view  of  face  of 
a  12  mm.  embryo;   B,  of  a  14  mm.  embryo. 

(Fig.  145).  At  the  end  of  the  fourth  week,  the  lateral  and  median  nasal  processes 
have  developed.  During  the  sixth  week,  the  maxillary  processes  fuse  with  the 
nasal  processes,  and  at  the  end  of  the  second  month  the  median  nasal  processes 
have  united.  The  mandibular  processes  fuse  at  the  sixth  week  and  from  them  a 
median  projection  is  developed  which  forms  the  anlage  of  the  chin. 


The  lips  begin  to  appear  as  folds  at  the  sixth  week.  As  the  median  nasal  processes  and 
the  maxillary  processes  take  part  in  their  development,  the  failure  of  these  parts  to  fuse  may 
produce  hare  lip.  The  lips  of  the  new-born  child  are  peculiar  in  that  their  proximal  surfaces 
are  covered  with  numerous  villi,  finger-like  processes  which  may  be  a  millimeter  or  more  in 
length. 


DKVKLOPMEXT   OF    THE    HARD    PALATE 


!55 


The  external  car  is  developed  around  the  first  branchial  cleft  by  the  appearance  of  small 
tubercules  which  form  the  auricle.  The  cleft  itself  becomes  the  external  auditory  meatus  and 
the  concha  of  the  ear.      (For  the  development  of  the  external  ear  see  Chapter  Xll). 


Fig.  145. — Development  of  the  face  of  the  human  embryo  (His).  A,  embryo  of  about  twenty-nine 
days.  The  median  frontal  process  differentiating  into  median  nasal  processes  or  processus  globulares, 
toward  which  the  maxillary  processes  of  first  visceral  arch  are  extending.  B,  embryo  of  about  thirty- 
four  days:  the  globular,  lateral  nasal,  and  maxillary  processes  are  in  apposition;  the  primitive  naris  is 
now  better  defined.  C,  embryo  of  about  the  eighth  week:  immediate  boundaries  of  mouth  are  more 
definite  and  the  nasal  orifices  are  partly  formed,  external  ear  appearing.  D,  embryo  at  end  of  second 
month. 


DEVELOPMENT  OF  THE  HARD  PALATE 
This  may  be  studied  advantageously  in  pig  embryos  of  two  stages:  (a)  20 
to  25  mm.  long;  (b)  28  to  35  mm.  long.  In  the  first  stage,  the  jaws  are  close 
together  and  the  mandible  usually  rests  against  the  breast  region.  The  palatine 
processes  are  separated  by  the  tongue  and  are  directed  ventrad  (Fig.  146  A). 
In  embryos  of  26  to  28  mm.,  the  jaws  open  and  the  tongue  lies  ventral  to  the 
palatine  processes  which  now  approach  each  other  in  a  horizontal  plane  (Fig. 
146  B).      Dissections  may  be  made  by  carrying  a  shallow  incision  from  the 


THE   DISSECTION    OF   PIG   EMBRYOS    FOR   STUDY 


angle  of  the  mouth  back  to  the  external  ear  on  each  side.  The  incisions  are  then 
continued  through  the  neck  in  a  plane  parallel  to  the  hard  palate.  Before  mount- 
ing the  preparation,  remove  the  top  of  the  head  by  a  section  cutting  through  the 
eyes  and  nostrils  parallel  to  the  first  plane  of  section.  Transverse  sections 
through  the  snout  may  also  be  prepared  to  show  the  positions  of  tongue  and 
palatine  folds  before  and  after  the  fusion  of  the  latter. 

In  the  human  embryo  of  two  and  a  half  months,  three  palatine  anlages  are 


Nasal  septum 

Tongue 
Lat.  palatine  process  —X 


Xasal  septum 
Lat.  palatine  process 


Mandible 


Turbinate  cnlage 


Tongue 


Fig.  146. — Sections  through  the  jaws  of  pig  embryos  to  show  development  of  the  hard  palate.     A,  22 

mm;    B,  34  mm.      X  8. 


developed:  a  small  median  process  developed  from  the  fused  median  nasal  pro- 
cesses, and  paired  lateral  palatine  processes  developed  from  the  maxillary  pro- 
cesses, and  extending  from  the  line  of  fusion  of  the  median  nasal  process  and  of 
the  maxillary  process  caudally  along  the  wall  of  the  pharynx  (Fig.  148).  In 
pig  embryos  (Fig.  147  A  and  B),  the  median  process  forms  a  single  heart-shaped 
structure.  The  lateral  palatine  processes  lie  at  first  lateral  and  ventral  to  the 
dorsum  of  the  tongue  and  their  edges  are  directed  ventrad  and  mesially  (Fig. 
146  A).     Before  these  processes  can  fuse,  the  tongue  is  withdrawn  from  between 


DEVELOPMENT  OF  THE  HARD  PALATE 


157 


them  owing  to  a  change  in  the  position  of  the  mandible  due  to  the  development 
of  its  arch  (Fig.  146  B).     With  the  withdrawal  of  the  tongue  the  edges  of  the 


Median  palatine 
process 
Lateral  palatine 
process 
Int.  choanal 


Oral  cavity 


Med.  palatine 
process 

Raphe  of  lat. 
palatine  process 


Nasal  passage 

A  nlage  of  inula 


Fig.  147. — Dissections  to  show  the  development  of  the  hard  palate  in  pig  embryos.  A,  ventral 
view  of  palatine  processes  of  a  22  mm.  pig  embryo,  the  mandible  having  been  removed;  B,  same  of  35 
mm.  embryo  showing  fusion  of  palatine  processes. 


/>•;'•'   /'•;;■ 


palatine  folds  are  approximated  and  soon  fuse,  thus  cutting  off  the  nasal  passages 

from  the  primitive  oral  cavity  dorsad  (Fig.  147  B).     At  the  point  in  the  median 

line  where  the  lateral  and  median 

palatine  processes  meet,  fusion  is 

not  complete,  leaving  the  incisive 

fossa,  and  laterad  between  the 

two   processes   openings    persist 

for  some  time,  which  are  known 

as  the  incisive  canals  (of  Sten- 

son). 

After  the  withdrawal  of  the 
tongue,  the  lateral  palatine  pro- 
cesses take  up  a  horizontal  posi- 
tion and  their  edges  are  approxi- 
mated, because  the  cells  on  the 
ventral  sides  of  the  folds  prolifer- 
ate more  rapidly  than  those  of 
the  dorsal  side  (Schorr,  Anat. 
Hefte,  Bd.  36,  1908).     That  the 

change  in  position  of  the  palatine  folds  is  not  mechanical,  but  due  to  unequal 
growth,  may  be  seen  in  Fig.  149,  a  section  through  the  palatine  folds  of  a  pig 


Fig.  148. — The  roof  of  the  mouth  of  a  human  embryo 
about  two  and  a  half  months  old.  showing  the  develop- 
ment of  the  palate,  p.g.,  processus  globularis;  p.g.' .  pala- 
tine process  of  processus  globularis;  mx,  maxillary  pro- 
cess; mx',  palatine  fold  of  maxillary  process.  Close  to 
the  angle  between  this  and  the  palatine  process  of  the 
processus  globularis  on  each  side,  the  primitive  choansc. 
(After  His.) 


158 


THE    DISSECTION    OF    PIG   EMBRYOS    FOR   STUDY 


embryo  which  shows  the  right  palatine  fold  in  a  horizontal  position,  although  the 
left  fold  projects  ventral  to  the  dorsum  of  the  tongue.  A  region  of  cellular  pro- 
liferation may  be  seen  on  the  under  side  of  each  process. 

At  the  end  of  the  second  month  the  palatine  bones  begin  to  develop  in  the 
lateral  palatine  folds  and  thus  form  the  hard  palate.  Caudally  the  bones  do  not 
develop  and  this  portion  of  the  folds  forms  the  soft  palate  and  the  uvula.     The  un- 


Nasal  septum 


Fig.  149. — Section  through  the  jaws  of  a  25  mm.  pig  embryo  to  show  the  change  in  the  position  of  the 
palatine  processes  with  reference  to  the  tongue. 

fused  backward  prolongations  of  the  palatine  folds  give  rise  to  the  arcus  pharyn- 
go-palatini,  which  is  taken  as  the  boundary  line  between  the  oral  cavity  proper 
and  the  pharynx  in  adult  anatomy.  , 

The  lateral  palatine  processes  occasionally  fail  to  unite  in  the  middle  line,  producing  a 
defect  known  as  cleft  palate.  The  extent  of  the  defect  varies  considerably,  in  some  cases  involv- 
ing only  the  soft  palate,  while  in  other  cases  both  soft  and  hard  palates  are  cleft. 


THE  DEVELOPMENT  OF  THE  TONGUE 
The  tongue  develops  as  two  distinct  portions,  the  body  and  the  root,  separated 
from  each  other  by  a  V-shaped  groove,  the  sulcus  terminalis.  Its  development 
may  be  studied  from  dissections  of  pig  embryos  6,  9  and  13  mm.  long.  As  the 
pharynx  is  bent  nearly  at  right  angles,  it  is  necessary  to  cut  away  its  roof  by  two 
pairs  of  sections  passing  in  different  planes.  The  first  plane  of  section  cuts 
through  the  eye  and  first  two  branchial  arches  just  above  the  cervical  sinus  (Fig. 
150,  I).  From  the  surface,  the  razor  blade  should  be  directed  obliquely  dorsal 
in  cutting  toward  the  median  line.  Cuts  in  this  plane  should  be  made  from  either 
side.  In  the  same  way  make  sections  on  each  side  in  a  plane  forming  an  obtuse 
angle  with  the  first  section  and  passing  dorsal  to  the  cervical  sinus  (Fig.  150,  II). 
Now  sever  the  remaining  portion  of  the  head  from  the  body  by  a  transverse  sec- 


THE  DEVELOPMENT  OF  THE  TONGUE 


159 


tion  in  a  plane  parallel  to  the  first  (Fig.  150,  III).     Place  the  ventral  portion  of 

the  head  in  a  watch-glass  of  alcohol  and,  under  the  dissecting  microscope,  remove 

that  part  of  the  preparation  cranial  to  the 

mandibular  arches.     Looking  down  upon  the 

floor  of  the  pharynx,  remove  any  portions  of 

the  lateral  pharyngeal  wall  which  may  still 

interfere  with  a  clear  view  of  the  pharyngeal 

arches  as  seen  in  Fig.  151.    Permanent  mounts 

of  the  three  stages  mentioned  above  may  be 

made  and  used  for  study  by  the  student. 

In  both  human  and  pig  embryos,  the 
body  of  the  tongue  is  developed  from  three 
anlages  which  are  formed  in  front  of  the 
second    branchial    arches.      These    are    the 

median,  somewhat  triangular  tubcrculum  impar,  and  the  paired  lateral  thicken- 
ings of  the  mandibular  arches,  both  of  which  are  present  in  human  embryos 
of  5  mm.  (Fig.  152  A).     At  this  stage,  a  median  ventral  elevation  formed  by 


Fig.  150. — Lateral  view  of  the  head 
of  a  7  mm.  pig  embryo.  The  three 
lines  indicate  the  planes  of  sections  to  be 
made  in  dissecting  the  tongue  as  de- 
scribed in  the  text. 


Br.  arch  I 


Tuberculum  impar 

Br.  arch  II 

Br.  arch  III 
Br.  arch  TV 

Arytenoid  ridge ! 


Lateral  lingual  anlage 


Br.  arch  I 
Lateral  lingual  anlage 

Br.  arch  III-^ 
Br.  arch  IV 
Arytenoid  ridge 


Tuberculum  impar 
Br.  arch  II 

Epiglottis 
Glottis 


Fig.  151. — Dissections  showing  the  development  of  the  tongue  in  pig  embryos.     A,  9  mm.  embryo; 

B,  13  mm.  embryo. 


i6o 


THE   DISSECTION   OF   PIG  EMBRYOS   FOR   STUDY 


the  union  of  the  second  branchial  arches  forms  the  copula.  This,  with  the  por- 
tions of  the  second  arches  lateral  to  it,  forms  later  the  base  or  root  of  the  tongue. 
Between  it  and  the  tuberculum  impar  is  the  point  of  evagination  of  the  median 
thyreoid  gland.  The  copula  also  connects  the  tuberculum  impar  with  a  rounded 
prominence  which  is  developed  in  the  mid- ventral  line  from  the  bases  of  the  third 
and  fourth  branchial  arches.  This  is  the  anlage  of  the  epiglottis.  In  later  stages 
(Fig.  151  A  and  B)  the  lateral  mandibular  anlages  increase  rapidly  in  size,  are 
bounded  laterally  by  the  linguo-alveolar  grooves,  and  fuse  with  the  tuberculum 
impar  which  lags  behind  in  development  and  is  said  to  form  the  median  septum 
of  the  tongue.  According  to  Hammar,  it  completely  atrophies.  The  epiglottis 
becomes  larger  and  concave  on  its  ventral  surface.     Caudal  to  it,  and  in  early 


Lateral  tongue      Thyreoid 
swellings      diverticulum 


Lateral  tongue  swellings 


Entrance  to 
larynx 


ntrance  to 
larynx 
rytenoid 
swellings 


Fig.  152. — The  development  of  the  tongue  in  human  embryos.    A,  5  mm.;  B,  7  mm.  (modified  from 

Peters). 


stages  continuous  with  it,  are  two  thick  rounded  folds,  the  arytenoid  folds.     Be- 
tween these  is  the  slit-like  glottis  leading  into  the  larynx  (see  p.  174). 

The  musculature  of  the  tongue  is  supplied  chiefly  by  the  hypoglossal  nerve,  and  both  nerve 
and  muscles  develop  caudal  to  the  branchial  region  in*which  the  tongue  develops.  The  mus- 
culature migrates  cephalad  and  gradually  invades  the  branchial  region  beneath  the  mucous 
membrane.  At  the  same  time,  the  tongue  may  be  said  to  extend  caudad  until  its  root  is  cov- 
ered by  the  epithelium  of  the  third  and  fourth  branchial  arches.  This  is  shown  by  the  fact 
that  the  sensory  portions  of  the  nn.  trigeminus  and  facialis,  the  nerves  of  the  first  and  second 
arches,  supply  the  body  of  the  tongue,  while  the  nn.  glossopharyngeus  and  vagus,  the  nerves  of 
the  third  and  fourth  arches,  supply  the  root  and  the  caudal  portion  of  the  body  of  the  tongue. 

In  embryos  of  50  to  60  mm.  the  fungiform  and  filiform  papillae  may  be  dis- 
tinguished as  elevations  of  the  epithelium.  Taste  buds  appear  in  the  fungiform 
papillae  of  100  mm.  embryos  and  are  much  more  numerous  in  the  fetus  than  in 
the  adult.     The  vallate  papillae  (Fig.  153  A)  appear  at  go  mm.  as  a  V-shaped  epi- 


DEVELOPMENT    OF    THE    SALIVARY    GLANDS 


161 


thelial  ridge,  the  apex  of  the  V  corresponding  to  the  site  of  the  median  thyreoid 
evagination.  At  intervals  along  the  epithelial  ridges  circular  epithelial  down- 
growths  develop  which  take  the  form  of  inverted  and  hollow  truncated  cones 
(Fig.  153  B).  During  the  fourth  month  circular  clefts  appear  in  the  epithelial 
downgrowths,  thus  separating  the  walls  of  the  vallate  papilla;  from  the  surround- 
ing epithelium  and  forming  the  trench  from  which  this  type  of  papilla  derives 
its  name.  At  the  same  time,  lateral  outgrowths  arise  from  the  bases  of  the  epi- 
thelial cones,  hollow  out  and  form  the  ducts  and  glands  of  Elmer.  The  taste 
buds  of  the  vallate  papillae  are  also  formed  early,  appearing  in  embryos  of  three 
months. 


Fig.   153. — Diagrams   showing   the   development  of  the  vallate   papilhe  of   the  tongue  (Graberg   in 

McMurrich's  "Human  Body"). 


DEVELOPMENT  OF  THE  SALIVARY  GLANDS 
The  glands  of  the  mouth  are  all  regarded  as  derivatives  of  the  ectodermal 
epithelium.  Of  the  salivary  glands,  the  parotid  is  the  first  to  appear.  Its  anlage 
has  been  observed  in  8  mm.  embryos  as  a  furrow  in  the  floor  of  the  alveolo-buccal 
groove.  The  furrow  elongates  and,  in  embryos  of  17  mm.,  separates  from  the 
epithelial  layer,  forming  a  tubular  structure  which  opens  into  the  mouth  cavity 
near  the  cephalic  end  of  the  original  furrow.  The  tube  grows  back  into  the 
region  of  the  external  ear,  branches  and  forms  the  gland  in  this  region,  while  the 
unbranched  portion  of  the  tube  becomes  the  parotid  duct  (Hammar,  Anat. 
Anzeiger,  Bd.  19,  1901). 

The  submaxillary  gland  arises  as  an  epithelial  ridge  in  the  alveolo-lingual 
groove,  its  cephalic  end  located  caudal  to  the  frenulum  of  the  tongue.  The 
caudal  end  of  the  ridge  soon  begins  to  separate  from  the  epithelium  and  extends 
caudad  and  ventrad  into  the  submaxillary  region  where  it  enlarges  and  branches 
to  form  the  gland  proper,  its  cephalic  unbranched  portion  persisting  as  the  duct 
which  soon  hollows  out. 


The  sub-lingual  and  alveolo-lingual  glands  develop  as  several  solid  evaginations  of  epi- 
thelium from  the  alveolo-lingual  groove,  appearing  from  the  eighth  to  the  twelfth  week  (Fig. 


162 


THE   DISSECTION   OF   PIG  EMBRYOS   FOR   STUDY 


157).     Of  the  alveolo-lingual  glands  nine  or  ten  may  develop  on  either  side  in  embryos  of  40 
mm.     (McMurrich  in  Keibel  and  Mall,  vol.  2,  p.  348-349.) 

The  branched  anlages  of  the  salivary  glands  are  at  first  solid  and  hollow  out  peripherally. 
The  glands  continue  growing  and  enlarging  until  after  birth.  Mucin  cells  may  be  distinguished 
by  the  sixteenth  week  and  acinus  cells  in  the  parotid  glands  at  five  months  (McMurrich). 


THE  DEVELOPMENT  OF  THE  TEETH 

The  development  of  the  teeth  is  described  in  all  the  standard  textbooks  of 
histology  and  only  a  brief  account  of  their  origin  and  structure  will  be  given  here. 
The  enamel  organs,  which  give  rise  to  the  enamel  of  the  teeth  and  are  the  moulds, 
so  to  speak,  of  the  future  teeth,  are  of  ectodermal  origin.  There  first  appears  in 
embryos  of  10  to  12  mm.  an  ectodermal  downgrowth,  the  dental  ridge  or  lamina 
on  the  future  alveolar  portions  of  the  upper  and  lower  jaws  (Fig.  154).      These 

uL. 


Fig.  154. — Early  stages  in  the  development  of  the  teeth.     A,  17  mm.;   B,  41  mm.  (Rose).     LF.,  LFL., 
labial  groove;   Pp.,  dental  papilla;    UK.,  lower  jaw;    uL.,  lower  lip;   ZL.,  dental  ridge. 


laminae  parallel  and  are  mesial  to  the  labial  grooves,  being  directed  obliquely 
toward  the  tongue.  At  intervals,  on  each  curved  dental  ridge  or  lamina  a  series 
of  thickenings  develop,  the  anlages  of  the  enamel  organs  (Fig.  155).  Soon  the 
ventral  side  of  each  enamel  organ  becomes  concave  (embryos  of  40  mm.)  forming 
an  inverted  cup  and  the  concavity  is  occupied  by  dense  mesenchymal  tissue,  the 
dental  papilla  (Figs.  154  B  and  156).  An  enamel  organ  with  dental  papilla 
forms  the  anlage  of  each  decidual  or  milk  tooth.  Ten  such  anlages  are  present  in 
the  upper  jaw  and  ten  in  the  lower  jaw  of  a  40  mm.  embryo.  The  connection  of 
the  dental  anlages  with  the  dental  ridge  is  eventually  lost.  The  position  of  the 
tooth  anlage  between  the  tongue  and  lip  is  shown  in  Fig.  157. 

The  anlages  of  those  permanent  teeth  which  correspond  to  the  decidual,  or 
milk  teeth,  are  developed  in  the  same  way  along  the  free  edge  of  the  dental 
lamina  median  to  the  decidual  teeth.  In  addition,  the  anlages  of  three  perma- 
nent molars  are  developed  on  each  side,  both  above  and  below  from  a  backward  or 


THE    DEVELOPMENT   OF   THE   TEETH 


163 


aboral  extension  of  the  dental  lamina,  entirely  free  from  the  oral  epithelium. 
The  anlages  of  the  first  permanent  molars  appear  at  seventeen  weeks  (180  mm.), 
those  of  the  second  molars  at  six  months  after  birth,  while  the  anlages  of  the  third 
permanent  molars  or  wisdom  teeth  are  not  found  until  the  fifth  year.  The  per- 
manent dentition  of  thirty-two  teeth  is  then  complete. 

The  internal  cells  of  the  enamel  organs  are  at  first  compact,  but  later  by  the 


Oral  epithelium 


Enamel  organs 


Dental 
groove 


Dental  lamina 


Necks  of 
enamel  organs 


Free  edge  of 
the  dental 

lamina 


Dental 

lamina 


Labial 
groove 


Milk 
molar  I 


Aboral 
prolonga- 
tion of 
dental 
lamina 


Fig.  155. — A,  B,  C,  D.  diagrams  showing  the  early  development  of  three  teeth.  One  of  the  teeth  is 
shown  in  vertical  section  (Lewis  and  Stohr).  E,  dental  lamina  and  anlages  of  the  milk  teeth  of  the  upper 
jaw  from  a  fetus  of  105  mm.  (Rose  in  Kollmann's  Handatlas). 


development  of  an  intercellular  matrix  the  cells  separate  forming  a  reticulum 
resembling  mesenchyma  and  termed  the  enamel  pulp  (Fig.  156).  The  outer 
enamel  cells,  at  first  cuboidal,  flatten  out  and  later  form  a  fibrous  layer.  The 
inner  enamel  cells  bound  the  cup-shaped  concavity  of  the  enamel  organ.  Over 
the  crown  of  the  tooth  these  cells,  the  ameloblasts,  become  slender  and  columnar 
in  form,  producing  the  enamel  layer  of  the  tooth  along  their  basal  ends  (Fig.  158). 


i64 


THE   DISSECTION   OF   PIG  EMBRYOS    FOR   STUDY 


The  enamel  is  laid  down  first  as  an  uncalcified  fibrillar  layer  which  later  becomes 
calcified  in  the  form  of  enamel  prisms.    From  the  ends  of  the  cells  project  cutic- 


Denfal  lamina 


Epidermis 


Fig.  156. — Section  through  the  upper  first  decidual  incisor  tooth  from  a  65  mm.  human  embryo.     X  70. 


*f^*S*vi- 


Tip  of  tongue 


Submaxillary  duct 

Sublingual  duct 


Epidermis  of  lip 


Enamel  organ  of  tooth 
Dental  papilla 

Meckel's  cartilage 
Bone  of  mandible 


Fig.  157. — Parasagittal  section  through  the  mandible  and  tongue  of  a  65  mm.  human  embryo  showing 
the  position  of  the  anlage  of  the' first  incisor  tooth.     X  14- 


ular  fibers  known  as  Tomes'  processes  (Fig.  158).     The  enamel  is  formed  first 
at  the  top  of  the  crown  of  the  tooth  and  extends  toward  the  root  over  the  crown. 


TIIK    1)K\ KI.OPMKN'T    OF    Till'.    TEETH 


i6« 


The  enamel  cells  about  the  future  root  of  the  tooth  remain  cuboidal  or  low  col- 
umnar in  form,  come  into  contact  with  the  outer  enamel  cells  and  the  two  layers 
constitute  the  epithelial  sheath  of  the  root  which  does  not  produce  enamel  prisms. 
The  Dental  Papilla. — The  outermost  cells  of  the  dental  papilla  at  the  end 
of  the  fourth  month  arrange  themselves  as  a  definite  layer  of  columnar  epithelium. 
Since  they  produce  the  dentine,  or  dental  bone,  these  cells  are  known  as  odonto- 
blasts. When  the  dentine  layer  is  developed,  the  odontoblast  cells  remain  in- 
ternal to  it  and  branched  processes  from  them  (the  dental  fibers  of  Tomes)  extend 
into  the  dentine  and  form  the  dental  canaliculi.  Internal  to  the  odontoblast 
layer  the  mesenchymal  cells  differentiate  into  the  dental  pulp,  popularly  known 


Fig.  158. — Section  through  a  portion  of  the  crown  of  a  developing  tooth  showing  the  various  layers 
(Tourneux  in  Heisler).  1,2,  cells  of  enamel  pulp;  j,  ameloblast  layer  of  enamel-forming  cells;  4,  5, 
enamel  prisms;  6,  layer  of  dentine  containing  processes  of  7,  odontoblast  cells;   S,  cells  of  dental  pulp. 


as  the  "nerve"  of  the  tooth.  This  is  composed  of  a  framework  of  reticular  tissue 
in  which  are  found  blood-vessels,  lymphatics  and  nerve  fibers.  The  odonto- 
blast layer  persists  throughout  life  and  continues  to  secrete  dentine  so  that 
eventually  the  root  canal  may  be  obliterated. 

Dental  Sac. — The  mesenchymal  tissue  surrounding  the  anlage  of  the  tooth 
gives  rise  to  a  dense  outer  layer  and  a  more  open  inner  layer  of  fibi  .mective 

tissue.  These  layers  form  the  dental  sac  (Fig.  159).  Over  the  root  of  the  tooth 
a  layer  of  osteoblasts  or  bone  forming  cells  develops,  and,  the  epithelial  sheath 
formed  by  the  enamel  layers  having  disintegrated,  these  osteoblasts  deposit  about 
the  dentine  a  layer  of  bone  which  is  known  as  the  substantia  ossea  or  cement. 
The  cement  layer  contains  typical  bone  cells  but  no  Haversian  canals.     As  the 


i66 


THE   DISSECTION   OF   PIG  EMBRYOS   FOR   STUDY 


teeth  grow  and  fill  the  alveoli  the  dental  sac  becomes  a  thin  vascular  layer  con- 
tinuous externally  with  the  alveolar  periosteum,  internally  with  the  periosteum  of 
the  cement  layer  of  the  tooth. 


Dental  sac 


Outer  I  aver  Inner  layer 


r^hm     :,        S^lHllii 


Outer  enamel  cells 

Enamel  pulp 

m^Inner  enamel  cells 
& 

Enamel 


Dentine 


Odontoblasts  /^j^SS 


Epithelial  sheath 


Dental  papilla  {future  pulp) 

Blood-vessel 
Bony  trabecula  of  the  lower 


Fig.  159. — Longitudinal  section  of  a  deciduous  tooth  of  a  newborn  dog.  X  42-  The  white 
spaces  between  the  inner  enamel  cells  and  the  enamel  are  artificial,  and  due  to  shrinkage  (Lewis 
and  Stohr). 


When  the  crown  of  the  tooth  is  fully  developed  the  enamel  organ  disinte- 
grates, and  as  the  roots  of  the  teeth  continue  to  grow  their  crowns  approach  the 
surface  and  break  through  the  gums.     The  periods  of  eruption  of  the  various 


Permanent  second  molar 


Deciduous  molars 
Mandibular  canal 

Permanent  first  molar 


Permanent  premolars 

Permanent  canine 


Mental  foramen 
Permanent  incisors 


Fig.  160. — The  skull  of  a  five-year-old  child  showing  positions  of  the  decidual  and  permanent  teeth 
(Sobotta-McMurrich.  Atlas  of  Human  Anatomy). 


TILE    DEVELOPMENT    OF    THE    TEETH  167 

milk  or  decidual  teeth  vary  with  race,  climate  and  nutritive  conditions.     Usually 
the  teeth  are  cut  in  the  following  sequence: 

[dual  'ik  Mm.k  Teeth 

Median  Incisors sixth  to  eighth  month. 

Lateral  Incisors eighth  to  twelfth  month. 

First  Molars twelfth  to  sixteenth  month. 

Canines^- seventeenth  to  twentieth  month. 

Second  Molars twentieth  to  twenty-fourth  month. 

The  permanent  teeth  are  all  present  at  the  fifth  year.  They  are  located 
mesial  to  the  decidual  teeth  (Fig.  160),  and,  before  the  permanent  teeth  begin 
to  erupt,  the  roots  of  the  milk  teeth  undergo  absorption,  their  dental  pulp  dies 
and  they  are  eventually  shed.  The  permanent  teeth  are  "cut"  as  follows: 
(McMurrich  in  Keibel  and  Mall,  vol.  2,  p.  354). 

First  Molars seventh  year 

Median  Incisors eighth  year. 

Lateral  Incisors ninth  year. 

First  Premolars tenth  year. 

Second  Premolars eleventh  year. 

c         j,r,         J   thirteenth  to  fourteenth  year. 

Second  Molars  J 


Third  Molars seventeenth  to  fortieth 


year. 


Dental  anomalies  are  frequent  and  may  consist  in  the  congenital  absence  of  some  or  all 
of  the  teeth,  or  in  the  production  of  more  than  the  normal  number.  Defective  teeth  are  fre- 
quently associated  with  hare  lip.  Cases  have  been  noted  in  which,  owing  to  defect  of  the 
enamel  organ,  the  enamel  was  entirely  wanting.  Many  cases  in  which  a  third  dentition  occurred 
have  been  recorded  and  occasionally  fourth  molars  may  be  developed  behind  the  wisdom  teeth. 

The  teeth  of  vertebrates  are  homologues  of  the  placoid  scales  of  elasmobranch  fishes 
(sharks).  The  teeth  of  the  shark  resemble  enlarged  scales,  and  many  generations  of  teeth  are 
produced  in  the  adult  fish.  In  some  mammalian  embryos  three  or  even  four  dentitions  are 
present.  The  primitive  teeth  of  mammals  are  of  the  canine  type  and,  from  this  conical  tooth, 
the  incisors  and  molars  have  been  differentiated. 


CHAPTER  VII 
THE  ENTODERMAL  CANAL  AND  ITS  DERIVATIVES:  THE  BODY  CAVITIES 

When  the  head-  and  tail-folds  of  the  embryo  develop,  there  are  formed  both 
cranially  and  caudally  from  the  spherical  vitelline  sac  blind  entodermal  tubes, 
the  fore-gut  and  hind-gut  respectively  (Fig.  161  A).     The  region  between  these 


Pharynx 

Pharyngeal 
membrane 

Pericardial 

cavity 

Fore-gut 

Hepatic 
diverticulum 

Yolk-stalk 


Hind-gal 

Cloacal 

membrane 

Allantois 

Cloaca 


Pharynx 

Pharyngeal 

membrane 

Thyreoid 

gland 

Pericardial 

cavity 

Fore-gut 

Hepatic 
diverticulum 


Yolk-stalk 

Allantois 

Cloacal 

membrane 

Cloaca 


Hind-gut 


Fig.  161. — Diagrams  showing  in  median  sagittal  section  the  alimentary  canal,  pharyngeal  and  cloacal 
membranes.     A,  2  mm.  embryo,  modified  after  His;   B,  2.5  mm.  embryo,  after  Thompson. 


intestinal  tubes,  open  ventrally  into  the  yolk-sac,  is  known  as  the  mid-gut.  As 
the  embryo  and  the  yolk-sac  at  first  grow  more  rapidly  than  the  connecting  re- 
gion between  them,  this  region  is  apparently  constricted  and  becomes  the  yolk- 
stalk,  or  vitelline  duct.  At  either  end  the  entoderm  comes  into  contact  ventrally 
with  the  ectoderm.     Thus  there  are  formed  the  pharyngeal  membrane  of  the  fore- 

168 


PHARYNGEAL   POUCHES  169 

gut,  the  cloacal  membrane  of  the  hind-gut.  In  2  mm.  embryos  the  pharyngeal 
membrane  separates  the  ventral  ectodermal  cavity,  or  slomodceum,  from  the 
pharyngeal  cavity  of  the  fore-gut.  Cranial  to  the  membrane  is  the  ectodermal 
diverticulum,  Rathkcs  pocket.  In  2.5  to  3  mm.  embryos  (Fig.  161  B)  the  pharyn- 
geal membrane  is  perforated  and  the  stomodaeum  and  pharynx  are  continuous. 
The  blind  termination  of  the  fore-gut  probably  forms  SccsseVs  pocket. 

The  fore-gut  later  forms  part  of  the  oral  cavity  and  is  further  differentiated 
into  the  pharynx  and  its  derivatives;  into  the  esophagus,  respiratory  organs, 
stomach,  duodenum,  jejunum  and  a  portion  of  the  ileum.  From  the  duodenum 
arise  the  liver  and  pancreas.  The  hind-gut,  beginning  at  the  attachment  of  the 
yolk-stalk  extends  caudally  to  the  cloaca,  into  which  opens  the  allantois  in  2  mm. 
embryos.  The  hind-gut  is  differentiated  into  the  ileum,  caecum,  colon  and  rec- 
tum. The  cloaca  is  subdivided  into  the  rectum  and  urogenital  sinus  (for  its  de- 
velopment see  Chapter  VIII).  At  the  same  time,  the  cloacal  membrane  is 
separated  into  a  urogenital  membrane  and  into  an  anal  membrane.  The  latter 
eventually  ruptures  and  this  opening  is  the  anus.  The  yolk-stalk  usually  loses 
its  connection  with  the  entodermal  tube  in  embryos  of  7.5  mm.  (Fig.  172). 

We  have  seen  how  the  palatine  processes  divide  the  primitive  oral  cavity 
into  the  nasal  passages  and  mouth  cavity  of  the  adult,  and  have  described  the 
development  of  the  tongue,  teeth  and  salivary  glands,  organs  derived  wholly  or 
in  part  from  the  ectoderm.  It  remains  to  trace  the  development  of  the  pharynx 
and  its  derivatives. 

PHARYNGEAL  POUCHES 

There  are  developed  early  from  the  lateral  wall  of  the  pharynx  paired  out- 
growths which  are  formed  in  succession  cephalo-caudad.  In  4  to  5  mm.  embryos, 
five  pairs  of  pharyngeal  pouches  are  present,  the  fifth  pair  being  rudimentary. 
At  the  same  time  there  appears  in  the  mid- ventral  wall  of  the  pharynx,  between 
the  first  and  second  branchial  arches,  a  small  rounded  prominence,  the  thyreoid 
anlage.  This  constricts  off  and  forms  a  stalked  vesicle  (Fig.  82).  Its  stalk,  the 
thyreo-glossal  duct,  opens  near  the  aboral  border  of  the  tuberculum  impar. 
Meantime,  the  pharynx  has  been  flattened  dorso-ventrally  and  broadened  later- 
ally and  cephalad  so  that  it  is  triangular  in  ventral  view  (Figs.  82  and  162). 

From  each  pharyngeal  pouch  develop  small  dorsal  and  large  ventral 
diverticula.  The  first  four  pouches  come  into  contact  with  the  ectoderm  of 
the  branchial  clefts,  fuse  with  it  and  form  the  closiiig  plates.  Only  occasionally 
do  the  closing  plates  become  perforate  in  human  embryos.     The  first  and  second 


170 


THE    ENTODERMAL    CANAL   AND    ITS    DERIVATIVES 


pharyngeal  pouches  soon  connect  with  the  pharyngeal  cavity  through  wide  open- 
ings. The  third  and  fourth  pouches  grow  laterad  and  their  diverticula  com- 
municate with  the  pharynx  through  narrow  ducts  in  10  to  12  mm.  embryos 
(Fig.  162).  When  the  cervical  sinus  is  formed  the  ectoderm  of  the  second,  third 
and  fourth  branchial  clefts  is  drawn  out  to  produce  branchial  and  cervical  ducts  and 
the  branchial  vesicle.  These  are  fused  at  the  closing  plates  with  the  entoderm  of 
the  second,  third  and  fourth  pharyngeal  pouches. 


Branchial  duct 2  Epithelial  body  of  3&  pouch 


Branchial 
cleft  I 


Cervical  sinus 


Cervical   dud 
Thymus  anlaqe  '' 

Epithelial  body  of 
¥h  pouch 

Trachea 


Pharyngeal 
pouch  1 

Pharyngeal 
pouch  Z 

Pharynqeal 
pouch  3 

Pharyngeal 
pouch  4- 


Siomach 


Dorsal  'pancreas 


Pharyngeal  pouch  6 
Esophagus 


Apical   bud  of  right  Lung 


Gall  bladder 
Duodenum 


Fig.  162. — A  reconstruction  of  the  pharynx  and  fore-gut  of  an  1 1.8  mm.  embryo  seen  in  dorsal  view  (after 
Hammar).     The  ectodermal  structures  are  stippled. 


The  first  and  second  pouches  soon  differ  from  the  others  in  form  and  give  rise 
to  an  entirely  different  type  of  permanent  structures.  With  the  broadening  of 
the  pharynx  the  first  two  pouches  acquire  a  common  opening  into  it,  the  primary 
tympanic  cavity.  The  first  pouch  later  differentiates  into  the  tympanic  cavity 
of  the  middle  ear  and  into  the  Eustachian  tube.  By  the  growth  and  lateral 
expansion  of  the  pharynx  the  second  pouch  is  taken  up  into  the  pharyngeal  wall, 
its  dorsal  angle  alone  persisting  to  be  later  transformed  into  the  palatine  tonsil. 


TILE    THYMUS 


171 


According  to  I  laminar  (Arch,  f.  mikr.  Anat.,  Bd.  61,  1903),  the  lateral  pharyngeal  recess 
(of  Rosenmueller)  is  not  a  persistent  portion  of  the  second  pouch  as  His  asserted.     Lymphocytes 
appear  in  the  Lymphoid  tissue  of  the 
tonsils  in  embryos  of  140mm.    They 
take   their  origin    in   the  mesoderm 
(Harnmar,  Maximo w). 


Foramen  caecum 


Palatine  tonsil 

Epithelial  bodies 


Tbyreo  qloaal  duct 


Thymus 
'aniages 

Post-branchial  body 


Thyreoid  antaoe 


THE  THYMUS 
The  third,  fourth  and  fifth 
pharyngeal  pouches  give  rise  to 
a  series  of  ductless  glands,  of 
which  the  thymus  is  the  most 
important.  The  thymus  anlage 
appears  in  10  mm.  embryos  as  a 
ventral  and  medial  prolongation 
of  the  third  pair  of  pouches  (Figs. 
162  and  163).  The  ducts  con- 
necting the  diverticula  with  the 
pharynx  soon  disappear  so  that 

the  thymus  anlages  are  set  free.    At  first  hollow  tubes,  they  soon  lose  their  cavities, 
their  lower  ends  enlarge  and  migrate  caudally  into  the  thorax  passing  usually  ven- 


Fig.  163. — Diagram  in  ventral  view  of  the  pharynx 
and  pharyngeal  pouches,  showing  the  origin  of  the  thymus 
and  thyreoid  glands  and  of  the  epithelial  bodies  (modified 
after  Groschuff  and  Kohn).  I,  II,  III,  IV,  and  V,  first, 
second,  third,  fourth  and  fifth  pharyngeal  pouches. 


Fig.  164. — Two  reconstructions  of  the  thymus  and  thyreoid  glands.  A.  in  a  human  embryo  of  26 
mm.;  B,  in  one  of  24  mm.  (after  Tourneux  and  Verdun).  In  A  the  thymus  lies  in  front,  in  B,  behind 
the  left  innominate  vein,  thyr.,  thyreoid;  thym.,  thymus;  par.  TV.,  parathyreoid  of  fourth  pouch; 
par.  III.,  parathyreoid  of  third  pouch;  pyr.,  pyramidal  lobe  of  thyreoid;  thyg.,  thyreoglossal  duct;  c.a.; 
carotid  artery;  j.v.,  jugular  vein. 


tral  to  the  left  vena  anonyma.  Their  upper  ends  become  attentuate  and  atrophy 
or  may  persist  as  an  accessory  thymus  lobe  (Kohn) .  The  enlarged  lower  ends  of 
the  anlages  form  the  body  of  the  gland,  which  is  thus  a  paired  structure  (Fig.  164). 


172  THE   ENTODERMAL   CANAL   AND   ITS   DERIVATIVES 

At  50  mm.  the  thymus  still  contains  solid  cords  and  small  closed  vesicles  of  ento- 
dermal  cells.  From  this  stage  on,  in  development,  the  gland  becomes  more  and 
more  lymphoid  in  character.  Its  final  position  is  in  the  thorax  dorsal  to  the  cranial 
end  of  the  sternum.  It  grows  under  normal  conditions  until  puberty,  after  which  its 
degeneration  begins.  This  process  proceeds  slowly  in  healthy  individuals,  rapidly 
in  case  of  disease.    The  thymus  may  function  normally  until  after  the  fortieth  year. 

It  is  now  generally  believed  that  the  entodermal  epithelium  of  the  thymus  is  converted 
into  reticular  tissue  and  thymic  corpuscles.  The  "lymphoid"  cells  are  regarded  by  Hammar 
and  Maximow  as  immigrant  lymphocytes  derived  from  the  mesoderm.  According  to  Stohr, 
they  are  not  true  lymphocytes  but  are  derived  from  the  thymic  epithelium.  Weill  (Arch.  f. 
mikr.  Anat.,  Bd.  83, pp.  305-360)  has  observed  the  development  of  granular  leucocytes  in  the 
human  thymus  gland. 

THE  EPITHELIAL  BODIES  OR  PARATHYREOIDS 

The  dorsal  diverticula  of  the  third  and  fourth  pharyngeal  pouches  each  give 
rise  to  a  small  mass  of  epithelial  cells  termed  an  epithelial  body  (Fig.  163).  Two 
pairs  of  these  bodies  are  thus  formed  and,  with  the  atrophy  of  the  ducts  of  the 
pharyngeal  pouches,  they  are  set  free  and  migrate  caudalward.  They  eventually 
lodge  in  the  dorsal  surface  of  the  thyreoid  gland,  the  pair  from  the  third  pouch 
lying  one  on  each  side  at  the  caudal  border  of  the  thyreoid  in  line  with  the  thymus 
anlages  (Fig.  164).  The  pair  of  epithelial  bodies  derived  from  the  fourth  pouches 
are  located  on  each  side  near  the  cranial  border  of  the  thyreoid.  From  their 
ultimate  relation  to  the  thyreoid  tissue  the  epithelial  bodies  are  often  termed 
parathyreoid  glands.  The  solid  body  is  broken  up  into  masses  and  cords  of  poly- 
gonal entodermal  cells  intermingled  with  blood-vessels.  In  post-fetal  life,  lumina 
may  appear  in  the  cell  masses  and  fill  with  a  colloid-like  secretion. 

The  ventral  diverticulum  of  the  fourth  pouch  is  a  rudimentary  thymic  an- 
lage.     It  soon  atrophies. 

The  ultimobranchial  body  is  the  derivative  of  the  fifth  pharyngeal  pouch 
(Fig.  163).  With  the  atrophy  of  the  duct  of  the  fourth  pouch  it  is  set  free  and 
migrates  caudad  with  the  parathyreoids.  It  forms  a  hollow  vesicle  which  has 
been  termed  the  lateral  thyreoid.  According  to  Grosser  (Keibel  and  Mall,  vol. 
2.  p.  467)  and  Verdun,  it  takes  no  part  in  forming  thyreoid  tissue  but  atrophies. 
The  term  lateral  thyreoid  when  applied  to  it  is  therefore  a  misnomer. 

THE  THYREOID  GLAND 
The  thyreoid  anlage  (Fig.  163)  is  bilobed  before  the  thyreoglossal  duct  dis- 
appears.    It  soon  loses  its  lumen  and  breaks  up  into  irregular  solid  cords  of  tissue 


LARYNX,  TRACHEA  AND  LUNGS 


173 


as  it  migrates  caudad.  It  takes  up  a  transverse  position  with  a  lobe  on  each  side 
of  the  trachea  and  larynx  (Fig.  164).  In  embryos  of  50  mm.,  lumina  appear  in 
the  more  peripheral  cords  which  break  up  into  hollow  or  solid  groups  of  cells,  the 
primitive  thyreoid  follicles  (Grosser). 

LARYNX,  TRACHEA  AND  LUNGS 
In  embryos  of  23  segments,  the  anlage  of  the  respiratory  organs  appears  as  a 
groove  in  the  floor  of  the  entodermal  tube  just  caudal  to  the  pharyngeal  pouches. 
This  groove  produces  an  external  ridge  on  the  ventral  wall  of  the  tube,  a  ridge 


Trachea, 

PespiraTory 
anlage 


Esophagus 


D 


Trachea. 

Apical  bud 

Primary 
bronchus 


Esophagus 


Trachea, 

Bronchus 
Ventral   bud 


Fig.  165. — Diagrams  of  stages  in  the  early  development  of  the  trachea  and  lungs  of  human  embryos 
(based  on  reconstructions  by  Bremer,  Broman,  Grosser,  and  Xaroth).  A,  2.$  mm.;  B,  4  mm.;  C,  B  in 
side  view;  D,  5  mm.;  E,  7  mm. 

which  becomes  larger  and  rounded  at  its  caudal  end  (Fig.  165V  The  laryngo- 
tracheal groove  and  the  ridge  are  the  anlages  of  the  larynx  and  trachea.  The 
rounded  end  of  the  ridge  is  the  unpaired  anlage  of  the  lungs. 

Externally  two  lateral  longitudinal  grooves  mark  off  the  dorsal  esophagus 
from  the  ventral  respiratory  anlages.  The  lung  anlage  rapidly  increases  in  size 
and  becomes  bilobed  in  embryos  of  4  to  5  mm.  The  lateral  furrows  become  deeper 
caudad  and  a  septum  is  formed  which  grows  cephalad,  separating  first  the  lung 
anlages  and  then  the  tracheal  tube  from  the  esophagus.  At  the  same  time  the 
laryngeal  portion  of  the  groove  and  ridge  is  developed  cranially  until  it  lies  be- 
tween the  third  and  fourth  branchial  arches  (Fig.  82).     At  5  mm.  the  respiratory 


174 


THE    ENTODERMAL    CANAL   AND    ITS   DERIVATIVES 


apparatus  consists  of  the  laryngeal  groove  and  ridge,  the  tubular  trachea  and  the 
two  lung  buds. 

The  Larynx. — In  embryos  of  5  to  6  mm.  the  oval  end  of  the  laryngeal  groove 
is  bounded  on  either  side  by  two  rounded  prominences,  the  arytenoid  swellings 
which,  continuous  orally  with  a  transverse  ridge,  form  the  furcula  of  His  (Fig. 
152  B).  The  transverse  ridge  becomes  the  epiglottis  and,  as  we  saw  in  connec- 
tion with  the  development  of  the  tongue,  it  is  derived  from  the  third  and 
fourth  branchial  arches.  In  embryos  of  15  mm.  the  arytenoid  swellings  are 
bent  near  the  middle  toward  the 
median  line.  Their  caudal  portions 
become  parallel,  while  their  cephalic  r.pli. 
portions  diverge  nearly  at  right  angles 
(Fig.  166).  The  opening  into  the 
larynx  thus   becomes  T-shaped   and 


o.l _. 


§ —  r.a.e. 


t-c.  .. 


f.i.a.. 


Fig.  166. — Entrance  to  larynx  in  a  forty-  to 
forty-two-day  human  embryo  (from  Kallius): 
t,  tuberculum  impar;  p,  pharyngo-epiglottic 
fold;  e,  epiglottic  fold;  l.e,  lateral  part  of  epi- 
glottis; at,  cuneiform  tubercle;  com,  cornicular 
tubercle. 


Fig.  167. — The  larynx  of  16  to  23  cm.  human 
embryos  (Soulie  and  Bardier).  From  a  dissection. 
b.l.,  base  of  tongue;  e,  epiglottis;  f.i.a.,  interary- 
tenoid  fissure;  r.a.e.,  plica  ary-epiglottica;  r.ph.e., 
plica  pharyngo-epiglottica;  o.l.,  orifice  of  larynx; 
t.c,  tuberculum  cuneiformis;  t. S.,  tuberculum  corni- 
culatum. 


ends  blindly,  as  the  laryngeal  epithelium  has  fused.  In  40  mm.  embryos  this 
fusion  is  dissolved,  the  arytenoid  swellings  are  withdrawn  from  contact  with 
the  epiglottis  and  the  entrance  to  the  larynx  becomes  oval  in  form  (Fig.  167). 
At  27  mm.  the  ventricles  of  the  larynx  appear  and  at  37  mm.  their  margins 
indicate  the  position  of  the  vocal  cords.  The  epithelium  of  the  vocal  cords 
is  without  cilia.  The  elastic  and  muscle  fibers  of  the  cords  are  developed  by 
the  fifth  month. 

At  the  eighth  week  the  cartilaginous  skeleton  of  the  larynx  is  indicated  by  a  surrounding 
condensation  of  mesenchyme.     The  cartilage  of  the  epiglottis  appears  relatively  late.    The 


LARYNX.  TRACHEA  AND  LUNGS 


175 


thyreoid  cartilage  is  formed  as  two  lateral  plates,  each  <>!'  which  has  two  centers  of  chondrili- 
cation.     These  plates  grow  ventrad  and  fuse  in  the  median  line. 

The  anlages  of  the  cricoid  and  arytenoid  cartilages  are  at  first  continuous.     Later,  sepa- 
rate cartilage  centers  develop  for  the  arytenoids.     The  cricoid  is  at  first  incomplete  <: 
but  eventually  forms  a  complete  ring.     The  cricoid  may  therefore  be  regarded  as  a  modified 
tracheal  ring.     The  comiculate  cartilages  are  portions  of  the  arytenoid  cartilages  and  separate 
from  them.     The  cuneiform  cartilages  are  derived  from  the  cartilage  of  the  epiglottis. 

The  Tracheal  Tube. — This  gradually  elongates  during  development  and  its 
columnar  epithelium  becomes  ciliated.  Muscle  fibers  and  the  anlages  of  the 
cartilaginous  rings  appear  at  17  mm.  The  glands  develop  as  ingrowths  of  the 
epithelium  during  the  last  five  months  of  pregnancy. 

The  Lungs. — Soon  after  the  lung  anlages  or  stem  buds  are  formed  (5  mm. 
embryos),  the  right  bronchial  bud  becomes  larger  and  is  directed  more  caudally 


FiG.  168. — Dorsal  and  ventral  views  of  the  lungs  from  a  human  embryo  of  five  weeks  (Merkel).     Ap, 
apical  bronchus;  Di,  D2,  etc.,  dorsal;  Vi,  V2,  etc.,  ventral  bronchi;  Jc,  infracardial  bronchus. 


(Fig.  165).  At  7  mm.  the  stem  bronchi  give  rise  to  two  bronchial  buds  on  the 
right  side,  to  one  on  the  left.  The  smaller  bronchial  bud  on  the  right  side  is  the 
apical  bud.  The  other  buds,  right  and  left,  are  known  as  ventral  bronchi.  There 
are  thus  formed  three  bronchial  rami  on  the  right  side,  two  on  the  left,  and  these 
correspond  to  the  primitive  lobes  of  the  lungs  (Fig.  168). 

On  the  left  side,  an  apical  bud  is  interpreted  as  being  derived  from  the  first  ventral  bron- 
chus. It  develops  later  and  remains  small  so  that  a  lobe  corresponding  to  the  upper  lobe  of 
the  right  lung  is  not  developed  in  the  left  lung  (Xaroth).  The  upper  lobe  of  the  left  lung  thus 
would  correspond  to  the  upper  and  middle  lobes  of  the  right  lung. 

The  bronchial  anlages  continue  to  branch  in  such  a  way  that  the  stem  bud 
becomes  the  main  bronchial  stem  (Fig.  168).  That  is.  the  branching  is  mono- 
podial,  not  dichotomous.  lateral  buds  being  given  off  from  the  stem  bud.  Only 
in  the  later  stages  of  development  has  dichotomous  branching  of  the  bronchi  and 


r76 


THE   ENTODERMAL  CANAL   AND   ITS   DERIVATIVES 


Parietal 
pleura 


the  formation  of  two  equal  buds  been  described.     Such  buds  formed  dichoto- 
mously  do  not  remain  of  equal  size  (Flint,  Amer.  Jour.  Anat.,  vol.  6,  1906-1907). 

The  entodermal  anlages 
of  the  lungs  and  trachea  are 
developed  in  a  median  mass 
of  mesenchyma  dorsal  and 
cranial  to  the  peritoneal 
cavity.  This  tissue  forms  a 
broad  mesentery  termed  the 
mediastinum  (Fig.  169).  The 
right  and  left  stem  buds  of 
the  lungs  grow  out  laterad, 
carrying  with  them  folds  of 
the  mesoderm.  The  branch- 
ing of  the  bronchial  buds 
takes  place  within  this  tissue 
which  is  covered  by  the 
mesothelium  which  lines  the 
body  cavity.  The  terminal 
branches  of  the  bronchi  are 
lined  with  entodermal  cells  which  flatten  out  and  form  the  respiratory  epithelium 
of  the  adult  lungs.     The  surrounding  mesenchyma  differentiates  into  the  muscle, 


Ductus 
Venosui 


Wall  of  umbilical  card 


Fig.  169. — Transverse  section  through  the  lungs  and  pleural 
cavities  of  a  10  mm.  human  embryo.     X  23. 


Fig.  170. — The  lungs  of  a  10.5  mm.  embryo  showing  the  pulmonary  arteries  and  veins  (His  from 
McMurrich's  "Human  Body").  Ap.,  pulmonary  artery;  Ep,  apical  bronchus;  Vp,  pulmonary  vein; 
/.  //,  primary  bronchi. 

connective  tissue  and  cartilage  plates  of  the  lungs,  trachea  and  bronchial  walls. 
Into  it  grow  blood-vessels  and  nerve   fibers.     When   the  pleural  cavities  are 


ESOPHAGUS,    STOMACH   AND   INTESTINE  1 77 

separated  from  the  pericardial  and  peritoneal  cavities,  the  mesothelium  covering 
the  lungs  with  the  connective  tissue  underlying  it  becomes  the  visceral  pleura. 
The  corresponding  layers  lining  the  thoracic  wall  form  the  parietal  pleura.  These 
layers  are  derived  respectively  from  the  visceral  (splanchnic)  and  parietal  (soma- 
tic) mesoderm  of  the  embryo. 

In  11  mm,  embryos  the  two  pulmonary  arteries,  from  the  sixth  pair  of  aortic 
arches,  course  lateral  then  dorsal  to  the  stem  bronchi  (Fig.  170).  The  right 
pulmonary  artery  passes  ventral  to  the  apical  bronchus  of  the  right  lung.  The 
single  pulmonary  vein  receives  two  branches  from  each  lung,  two  larger  veins 
from  each  lower  lobe,  two  smaller  veins  from  each  upper  lobe  and  the  middle  lobe 
of  the  right  side.  These  four  pulmonary  branches  course  ventrad  and  drain  into 
the  pulmonary  trunk.  When  this  common  stem  is  taken  up  into  the  wall  of  the 
left  atrium,  the  four  pulmonary  veins  open  directly  into  the  latter. 

According  to  Kolliker,  the  air  cells  of  the  lungs  begin  to  form  at  the  sixth  month  and  their 
development  is  completed  during  pregnancy.  Elastic  tissue  may  be  recognized  at  the  third 
month  in  the  walls  of  the  vessels  and  during  the  fourth  month  it  appears  in  the  largest  bronchi. 
The  abundant  connective  tissue  found  between  the  bronchial  branches  in  early  fetal  life  becomes 
reduced  in  its  relative  amount  as  the  alveoli  of  the  lungs  are  developed. 

Before  birth  the  lungs  are  relatively  small,  compact  and  possess  sharp  margins.  They 
lie  in  the  dorsal  portion  of  the  pleural  cavities.  After  birth  they  normally  fill  with  air,  expanding 
and  completely  filling  the  pleural  cavities.  Their  margins  become  rounded  and  the  compact 
fetal  lung  tissue  which  resembles  that  of  a  gland  in  structure  becomes  light  and  spongy,  owing 
to  the  enormous  increase  in  the  size  of  the  alveoli  and  blood-vessels.  Because  of  the  greater 
amount  of  blood  admitted  to  the  lungs  after  birth  their  weight  is  suddenly  increased. 

In  the  most  common  anomaly  involving  the  esophagus  and  trachea  the  former  is  divided 
transversely,  the  trachea  opening  into  the  lower  portion  of  the  esophagus,  while  the  upper 
portion  of  the  esophagus  ends  blindly.  According  to  Lewis  (in  Keibel  and  Mall,  vol.  2,  p.  367), 
the  anomaly  may  be  produced  by  the  abnormal  development  of  lateral  esophageal  grooves 
which  occlude  the  lumen  of  the  esophagus.  These  grooves,  though  small,  were  found  present 
in  4  mm.  human  embryos. 


ESOPHAGUS,  STOMACH  AND  INTESTINE 
Esophagus. — The  esophagus  in  4  to  5  mm.  embryos  is  a  short  tube,  flattened 
laterally,  and  extending  from  the  pharynx  to  the  stomach.  Its  epithelium  is 
composed  of  two  layers  of  columnar  cells.  The  esophagus  grows  rapidly  in  length 
and  in  7.5  mm.  embryos  its  diameter  decreases  both  relatively  and  absolutely 
(Forssner). 

In  embryos  of  from  8  to  16  mm.  its  laryngeal  end  is  crescent-shaped  and  concave  toward 
the  trachea.  Its  middle  portion  is  round  or  oval  and  opposite  the  bifurcation  of  the  trachea 
it  begins  to  enlarge  and  is  flattened  laterally.     Its  lumen  is  open  throughout  and  shows  from 


i78 


THE    ENTODERMAL    CANAL   AND    ITS    DERIVATIVES 


two  to  four  rows  of  nuclei.  In  20  mm.  embryos,  vacuoles  appearing  in  the  epithelium  give  the 
esophagus  the  appearance  of  having  several  lumina.  The  result  of  vacuole  formation  is  to  increase 
the  size  of  the  lumen.  In  later  stages  the  wall  of  the  esophagus  is  folded  and  ciliated  epithelial 
cells  appear  in  44  mm.  embryos.  The  number  of  cell  layers  in  the  epithelium  increases  until  at 
birth  they  number  nine  or  ten.  Glands  are  developed  as  ingrowths  from  the  epithelium.  The 
circular  muscle  layer  is  indicated  at  10  mm.  by  a  circular  layer  of  myoblasts,  but  the  longitudinal 
muscle  layer  does  not  form  a  definite  layer  until  55  mm.  (F.  T.  Lewis  in  Keibel  and  Mall,  vol.  2). 

Stomach. — The  stomach  appears  in  embryos  of  4  to  5  mm.  as  a  laterally 
flattened  fusiform  enlargement  of  the  fore-gut  caudal  to  the  lung  anlages  (Fig. 
171).     Its  epithelium  is  early  thicker  than  that  of  the  esophagus  and  is  sur- 
rounded by  a  thick  layer  of 


Pharynx 
Root  of  tongue 
Thyreoid 
Tip  of  tongue 


splanchnic  mesoderm.  It  is 
attached  dorsally  to  the  body 
wall  by  it's  mesentery,  the 
greater  omentum,  and  ventrally 
to  the  liver  by  the  lesser  omen- 
tum. The  dorsal  border  of  the 
stomach  enlarges  to  form  the 
fundus  and  greater  curvature,. 
The  dorsal  wall  grows  more 
rapidly  than  the  ventral  wall 
and  thus  produces  the  convex 
greater  curvature.  The  whole 
stomach  becomes  curved  and 
its  caudal  end  is  carried 
ventrad  and  to  the  right 
(Fig.  162).  This  forms  a 
ventral  concavity,  the  lesser 
curvature,  and  produces  the  first  flexure  of  the  duodenum.  The  rapid 
growth  of  the  gastric  wall  along  its  greater  curvature  also  causes  the  stomach 
to  rotate  upon  its  long  axis  until  its  greater  curvature  or  primitive  dorsal 
wall  lies  to  the  left,  its  ventral  wall,  the  lesser  curvature,  to  the  right  (Fig. 
162).  The  original  right  side  is  now  dorsal,  the  left  side  ventral  in  position, 
and  the  caudal  or  pyloric  end  of  the  stomach  is  ventral  and  to  the  right  of 
its  cardiac  or  cephalic  end.  The  whole  organ  extends  obliquely  across  the 
peritoneal  cavity  from  left  to  right.  The  change  in  position  progresses  rapidly 
and  is  already  completed  in  embryos  of  12  to  15  mm.  (beginning  of  the  second 
month).     The  rotation  of  the  stomach  explains  the  asymmetrical  position  of  the 


Rathke's  pocket 
Trachea 
Stomach 

Liver 

Dorsal  pancreas 

Hepatic  diverti- 
culum 

Yolk-stalk 
Allantois 
Mesonephric  duct 
Cloaca 

Hind-gut 

Fig.  171  A. — Median  sagittal  section  of  a  5  mm.  embryo  to 
show  the  digestive  canal  (modified  after  Ingalls). 


KSOI'IIAC.IS,    STOMACH     WD    INTKSTIXK 


179 


vagus  nerves  of  the  adult  organ,  the  left  nerve  supplying  t he  ventral  wall  of  the 
stomach,  originally  the  left  wall,  while  the  right  vagus  supplies  the  dorsal  wall, 
originally  the  right. 

Gastric  pits  are  indicated  in  16  mm.  embryos  and  at  100  mm.  gland  cells  of  the  gastric 
glands  are  differentiated.     These  undoubtedly  arise  from  the  gastric  epithelium,  according  to 


Tongui 


Rathke's 
pouch 


Yolk-stalk 

Allantoic 
stalk 


Cloaca 


Metan- 

ephros 

Mcson- 

ephric 

duct 


Laryngo- 
tracheal 
groove 


L.  lung 


Stomach 


Dorsal 
pancreas 


Allanlois 


Fig.  171  B. — Reconstruction  of  a  5  mm.  human  embryo  showing  the  entodermal  canal  and  its 
derivatives  (His  in  Kollmann's  Handatlas). 


Lewis.  The  cardiac  glands  are  developed  early  (01  mm.  embryos)  and,  according  to  Lewis, 
there  is  no  "evidence  in  favor  of  Bensley's  conclusion  that  the  cardiac  glands  are  decadent  .  .  . 
fundus  glands." 

At  10  mm.  the  stomach  wall  is  composed  of  three  layers,  entodermal  epithelium,  a  thick 
mesenchymal  layer  and  the  peritoneal  mesothelium.  At  16  mm.  the  circular  muscle  layer  is 
indicated  by  condensed  mesenchyma.     The  tunica  propria  forms  a  dense  layer  at  55  mm. 


i  So 


THE    ENTODERMAL    CANAL   AND    ITS    DERIVATIVES 


At  91  mm.  the  cardiac  region  shows  a  few  longitudinal  muscle  fibers,  which  become  distinct 
in  the  pyloric  region  at  240  mm. 

The  Intestine. — In  5  mm.  embryos  (Fig.  171  A),  the  intestine,  beginning  at 
the  stomach,  consists  of  the  duodenum  (from  which  are  given  off  the  hepatic 
diverticulum  and  ventral  pancreas),  and  the  cephalic  and  caudal  limbs  of  the 
intestinal  loop,  which  bends  ventrad  and  connects  with  the  yolk-stalk.     Caudally 


Rathke's  pocket 


Hypophysis 


Thyreoid 


Pericardium 


Allantois 


Cloacal  membrane 


Notochord 


Dorsal  pancreas 
Ventral  pancreas 


Ccecum 


Urogenital  sinus 


Peritoneal  cavity 
Tail-gut  \        Mcsonephric  duct 

Rectum 

Fig.  172. — Diagram  in  median  sagittal  section  showing  the  digestive  canal  of  a  9  mm.  human  embryo 

(adapted  from  Mall). 


the  intestinal  tube  expands  to  form  the  cloaca.     It  is  supported  from  the  dorsal 
body  wall  by  the  mesentery  (Fig.  171  B). 

From  5  to  9  mm.  the  ventral  bend  of  the  intestinal  loop  becomes  more  marked 
and  the  attachment  of  the  yolk-stalk  to  it  normally  disappears  (Fig.  172). 

The  attachment  of  the  yolk-stalk  may  persist  in  later  stages  (12  to  14  mm.  embryos, 
according  to  Keibel,  Elze  and  Thyng).  Also  in  20  per  cent,  of  adult  intestines  a  pouch  3  to  9 
cm.  long  is  found  where  the  yolk-stalk  was  formerly  attached.  This  pouch,  the  diverticulum 
of  the  ileum  or  Meckel' 's  diverticulum,  is  of  clinical  importance  as  many  cases  of  intestinal  occlu- 
sion in  infancy  are  due  to  its  presence. 


ESOPHAGUS,    STOMACH   AND    INTESTINE 


181 


At  the  stage  shown  in  Fig.  172,  the  dorsal  pancreatic  anlage  has  been  de- 
veloped from  the  duodenum  and,  in  the  caudal  limb  of  the  intestinal  loop,  there 
is  formed  an  enlargement  due  to  a  ventral  bulging  of  the  gut  wall  which  marks 
the  anlage  of  the  cacum  and  the  boundary  line  between  the  large  and  small  in- 
testine. The  caccal  anlage  later  differentiates  into  the  large  ccecum  and  distal 
vermiform  process  of  the  adult. 

Succeeding  changes  in  the  intestine  consist  (1)  in  its  torsion  and  coiling  due 


Brain 


Tip  of  tongue 
Thyreoid  gland 


Pericardium 

Gall-bladder 

Small  intestine 
Ccecum 


^  Hypophysis 

Foramen  ccecum 
{-Root  of  tongue 

Esophagus 

-Trachea 
.Notochord 
-Spinal  cord 


Urogenital  sinus 
Anal  membrane 


Rectum 


Fig.  173. — Diagrammatic  median  sagittal  section  of  a  17  mm.  human  embryo  showing  the  digestive  canal 

(modified  after  Mall). 


to  its  rapid  elongation  and  (2)  in  the  differentiation  of  its  different  regions.  As 
the  gut  elongates  in  9  to  10  mm.  embryos  the  intestinal  loop  rotates.  As  a  result, 
its  caudal  limb  lies  at  the  left  and  cranial  to  its  cephalic  limb  (Fig.  172).  At 
this  stage  the  intestinal  loop  enters  the  ccelom  of  the  umbilical  cord. 

The  small  intestine  soon  lengthens  rapidly  and  at  17  mm.  (Fig.  173)  forms 
loops  in  the  umbilical  cord.  Six  primary  loops  occur  and  may  be  recognized  in 
the  arrangement  of  the  adult  intestine  (Mall,  Bull.  Johns  Hopkins  Hosp.,  vol.  9, 
1898).     In  embryos  of  42  mm.  the  intestine  has  returned  from  the  umbilical 


182 


THE    ENTODERMAL    CANAL   AND    ITS    DERIVATIVES 


cord  into  the  abdominal  cavity  through  a  rather  small  aperture  and  the  ccelom 
of  the  cord  is  soon  after  obliterated. 

In  embryos  between  10  and  30  mm.  vacuoles  appear  in  the  wall  of  the  duodenum  and  epi- 
thelial septa  completely  block  the  lumen.  The  remainder  of  the  small  intestine  remains  open, 
although  vacuoles  form  in  its  epithelium.  Villi  appear  as  rounded  elevations  of  the  epithelium 
at  22.S  mm.  (Johnson).  They  begin  to  form  at  the  cephalic  end  of  the  jejunum,  and  at  130 
mm.  they  are  found  throughout  the  small  intestine  (Berry).  Intestinal  glands  appear  as 
ingrowths  of  the  epithelium  about  the  bases  of  the  villi.  They  develop  first  in  the  duodenum 
at  91  mm.  and  in  the  jejunum  at  130  mm.  The  duodenal  glands  (of  Brunner)  are  said  to  appear 
during  the  fourth  month  (Brand).  From  10  to  12.5  mm.  the  circular  muscle  layer  is  formed. 
The  longitudinal  muscle  layer  is  not  distinct  until  75  mm. 

The  impervious  duodenum  of  the  embryo  may  persist  as  a  congenital  anomaly,  and  we 
have  already  alluded  to  the  persistence  of  the  yolk-stalk  as  Meckel's  diverticulum. 


Fig.  174. — Three  successive  stages  showing  the  development  of  the  digestive  tube  and  the  mesen- 
teries in  the  human  fetus  (modified  from  Tourneux) :  1,  stomach;  2,  duodenum;  3,  small  intestine;  4, 
colon;  5,  vitelline  duct;  6,  caecum;  7,  great  omentum;  8,  mesoduodenum;  9,  mesentery;  10,  mesocolon. 
The  arrow  points  to  the  orifice  of  the  omental  bursa.     The  ventral  mesentery  is  not  shown  (Heisler). 


The  large  intestine,  as  we  have  seen  in  9  mm.  embryos,  forms  a  tube  ex- 
tending from  the  coecum  to  the  cloaca.  It  does  not  lengthen  so  rapidly  as  the 
small  intestine  and,  when  the  intestine  is  withdrawn  from  the  umbilical  cord  (at 
42  mm.),  its  cranial  or  caecal  end  lies  on  the  right  side  and  dorsal  to  the  small 
intestine  (Fig.  174).  It  extends  transversely  to  the  left  side  as  the  transverse 
colon,  then  bending  abruptly  caudad  as  the  descending  colon,  returns  by  its  iliac 
flexure  to  the  median  plane  and  forms  the  rectum. 

The  caecum  (Fig.  175)  is  differentiated  from  the  vermiform  process  at  65 
mm.  (Tarenetzky).  The  caecum  and  vermiform  process -make  a  U-shaped  bend 
with  the  colon  at  42  mm.,  and  this  flexure  gives  rise  to  the  ileo-ccecal  valve  (Toldt). 
In  stages  between  100  and  220  mm.  the  lengthening  of  the  colon  causes  the  caecum 


THE    LIVER 


183 


and  cephalic  end  of  the  colon  to  descend  toward  the  pelvis  (Fig.  174).  The 
ascending  colon  is  thus  formed  and  the  vermiform  appendix  takes  the  position 
which  it  occupies  in  the  adult.  The  development  of  the  mucous  membrane 
of  the  intestinal  tube  has  been  described  by  Johnson  (American  Journal  of 
Anatomy,  vols.  10,  14  and  16,  pp.  521-561;    187-233;    1-49). 


Ascending 

mesocolon 

Ascending 

colon 


Car  urn 


•ding 
colon 


Caecum 


Processus 
icrmiformis 

Processus 
vermiforr.iis 

Fig.  175. — The  caecum  of  a  human  embryo  of  5  cm.  (Kollmann).     A,  from  the  ventral  side;  B,  from  the 

dorsal   side. 


THE  LIVER 

In  embryos  of  2.5  mm.  the  liver  anlage  is  present  as  a  median  ventral  out- 
growth from  the  entoderm  of  the  fore-gut  just  cranial  to  the  yolk-stalk  (Fig. 
161  B).  Its  thick  walls  enclose  a  cavity  which  is  continuous  with  that  of  the  gut. 
The  liver  anlage  is  embedded  in  the  ventral  mesentery  which  lies  in  the  median 
line  between  the  fore-gut,  the  ventral  body  wall,  and  the  septum  transversum 
(Fig.  1 7 1  A) .  Thus,  from  the  first  the  liver  is  in  close  relation  to  the  septum  trans- 
versum and  later  when  the  septum  becomes  a  part  of  the  diaphragm  the  liver 
remains  attached  to  it. 

In  embryos  4  to  5  mm.  long,  solid  cords  of  cells  proliferate  from  the  ventral 
and  cranial  portion  of  the  liver  anlage.  These  cords  anastomose  and  form  a 
crescentic  mass  with  wings  extending  lateral  and  dorsal  to  the  gut  (Fig.  171  A). 


184 


THE   ENTODERMAL   CANAL  AND   ITS   DERIVATIVES 


Pa-.- 


This  mass,  a  network  of  solid  trabecular,  is  the  glandular  portion  of  the  liver. 

The  primitive,  hollow,  hepatic  diver- 
ticulum later  differentiates  into  the 
gall-bladder  and  the  large  biliary- 
ducts. 

Referring  to  Figs.  83  and  176,  it 
will  be  seen  that  the  liver  anlage  lies 
between  the  vitelline  veins  and  is  in 
close  proximity  to  them  laterally. 
The  veins  send  anastomosing 
branches  into  the  ventral  mesen- 
tery. The  trabeculae  of  the  expand- 
ing liver  grow  between  and  about 
these  venous  plexuses,  and  the  plex- 
uses in  turn  make  their  way  between 
and  around  the  liver  cords.  The 
vitelline  veins  on  their  way  to  the 
heart  are  thus  surrounded  by  the 
liver  and  largely  subdivided  into  a 
network  of  vessels  termed  sinusoids. 
The  endothelium  of  the  sinusoids  is  closely  applied  to  the  cords  of  liver  cells 
which,  in  the  early  stages,  con- 
tain no  bile  capillaries  (Fig.  177). 
For  the  transformation  of  the 
vitelline  veins  into  the  portal 
vein  and  for  the  relations  of  the 
umbilical  veins  to  the  liver  see 
Chapter  IX. 

The  glandular  portion  of 
the  liver  grows  rapidly  and  in 
embryos  of  7  to  8  mm.  is  con- 
nected with  the  primitive  hepa- 
tic diverticulum  only  by  a  single 
cord  of  cells,  the  hepatic  duct 
(Fig.  178  A).     That  portion  of 

the  hepatic  diverticulum  distal  to  the  hepatic  duct  is  now  differentiated  into  the 
terminal  solid  gall-bladder  and  its  cystic  duct.     Its  proximal  portion  forms  the 


Fig.  176. — The  liver  anlage  of  a  4  mm.  human 
embryo  (Bremer).  In.,  intestine;  Pa.,  pancreas; 
V.V.,  veins  in  contact  with  liver  trabeculae. 


Fig.  177. — The  trabeculae  and  sinusoids  of  the  liver 
in  section,  h.c,  trabecular  of  liver  cells;  Si.,  sinusoids 
(after  Minot).     X  300. 


THE    LIVER 


18: 


ductus  choledochns .  In  embryos  of  10  mm.  (Fig.  178  B)  the  gall-bladder  and 
ducts  have  become  longer  and  more  slender.  The  hepatic  duct  receives  a  right 
and  left  branch  from  the  corresponding  lobes  of  the  liver.  The  gall-bladder  is 
without  a  lumen  up  to  the  15  mm.  stage.  Later  its  cavity  appears  surrounded 
by  a  wall  of  high  columnar  epithelium. 

The  glandular  portion  of  the  liver  develops  fast  and  is  largest  relatively  at 
31  mm.  (Jackson,  Anat.  Record,  vol.  3,  pp.  361-396,  1909).  The  liver  tissue 
degenerates,  especially  in  the  peripheral  portion  of  the  left  lobe.  In  embryos  of 
two  months  the  liver  weighs  2  gm.;   at  birth  75  gm.;   in  the  adult  1500  gm. 


During  the  development  of  the  liver  the  endothelial  cells  of  the  sinusoids  become  stellate 
in  outline,  and  thus  form  an  incomplete  layer.     From  the  second  month  of  fetal  life  to  some 


Hepaii  c  ducr 

]ucfuS   choledochus 
pancreas 


brsal 
pancreas 


Ductus    chaledoc 
Ventral  pancreas 


Duodenum 


Gall  blade 

Cystic  duct 


Duct  of  dorsal 

pancreas 

Head  of  dorsal 

pancreas 

Duodenum 


Tail  of dorsa,f 
pancreas 


Fig.  178. — Reconstructions  showing  the  development  of  the  hepatic  diverticulum  and  pancreatic 
anlages.  A,  7.5  mm.  embryo,  X  36  (after  Thyng);  B,  10  mm.  embryo,  X  33  (specimen  loaned  by  Dr. 
H.  C.  Tracy). 


time  after  birth,  blood-cells  are  actively  developed  between  the  hepatic  cells  and  the  endothelium 
of  the  sinusoids.  The  hepatic  trabecular  are  mostly  solid  in  10  mm.  embryos.  At  22 
mm.  hollow  periportal  ducts  develop,  spreading  inward  from  the  hepatic  duct  along  the 
larger  branches  of  the  portal  vein.  These  ducts  form  a  plexus,  as  has  been  proved  by  injections. 
Lumina  bounded  by  five  or  six  cells  may  be  observed  in  some  of  the  liver  trabecular  of  10  mm. 
embryos  (Lewis).  In  44  mm.  embryos,  bile  capillaries  with  cuticular  borders  are  present, 
most  numerous  near  the  periportal  ducts  with  which  some  of  them  connect.  At  birth,  or 
shortly  after,  the  number  of  liver  cells  surrounding  a  bile  capillary  is  reduced  to  two,  three  or 
four. 

The  lobules,  or  vascular  units  of  the  liver,  are  formed,  according  to  Mall,  by  the  peculiar 
and  regular  manner  in  which  the  veins  of  the  liver  branch.  The  primary  branches  of  the  portal 
vein  extend  along  the  periphery  of  each  primitive  lobule,  parallel  to  similar  branches  of  the 
hepatic  veins  which  drain  the  blood  from  the  center  of  each  lobule  (Fig.  179).  As  development 
proceeds,  each  primary  branch  becomes  a  stem,  giving  off  on  either  side  secondary  branches 


i86 


THE   ECTODERMAL   CANAL  AND    ITS   DERIVATIVES 


which  bear  the  same  relation  to  each  other  and  to  new  lobules  as  did  the  primary  branches  to 
the  first  lobule.     This  process  is  repeated  until  thousands  of  liver  lobules  are  developed. 

Until  the  20  mm.  stage  the  portal  vein  alone  supplies  the  liver.  The  hepatic  artery  from 
the  cceliac  axis  comes  into  relation  first  with  the  hepatic  duct  and  gall-bladder.  Later,  it 
grows  into  the  connective  tissue  about  the  larger  bile  ducts  and  branches  of  the  portal  vein, 
and  also  supplies  the  capsule  of  the  liver. 

The  development  of  the  ligaments  of  the  liver  is  described  on  p.  200. 

Anomalies  of  the  liver  occur  chiefly  in  connection  with  the  gall-bladder  and  ducts.  The 
gall-bladder  may  be  absent  or  two  may  be  present.  Duplications  and  absence  of  the  hepatic 
duct  has  been  observed,  also  duplication  of  the  cystic  duct.  In  some  animals  (horse,  elephant) 
the  gall-bladder  is  normally  absent. 


fs 


A 


B 


Fig.  179. — Diagrams  of  three  successive  stages  of  the  portal  and  hepatic  veins  in  a  growing  liver. 
a,  Hepatic  side;  d,  portal  side;  b  and  c,  successive  stage  of  the  hepatic  vein;  e  and/,  successive  stages  of 
the  portal  vein  (Mall). 


^THE  PANCREAS 
Two  pancreatic  anlages  are  developed  almost  simultaneously  in  embryos  of 
3  to  4  mm.  The  dorsal  pancreas  arises  as  a  hollow  outpocketing  of  the  dorsal 
duodenal  wall  slightly  cranial  to  the  hepatic  diverticulum.  At  7.5 'mm.  it  is 
separated  from  the  duodenum  by  a  slight  constriction  (Fig.  178  A).  The  ventral 
pancreas  develops  in  the  inferior  angle  between  the  hepatic  diverticulum  and  the 
gut  (Lewis)  and  its  wall  is  continuous  with  both.  With  the  elongation  of  the 
ductus  choledochus  it  is  gradually  separated  from  the  intestine. 

The  ventral  pancreas  may  arise  directly  from  the  intestinal  wall.  In  cases  observed  by 
Debeyre,  Helly  and  Kollmann,  the  anlage  was  paired  and  in  other  embryos  a  paired  structure 
is  indicated. 


THE    PANCREAS 


l87 


Of  the  two  pancreatic  anlages,  the  dorsal  grows  more  rapidly  and  in  10  mm. 
embryos  forms  an  elongated  structure  with  irregular  nodules  upon  its  surface. 
Its  distal  portion  is  constricted  to  form  a  short  duct.  It  lies  in  the  greater  omen- 
tum between  the  duodenum  and  the  stomach.  The  ventral  pancreas  is  smaller 
and  develops  a  short  slender  duct  which  opens  into  the  ductus  choledochus. 
As  the  latter  elongates  it  bends  dorsad  and  to  the  right  of  the  intestine,  while  at 
the  same  time  the  stomach  and  intestine  rotate  to  the  right.  This  shifts  the  duct 
of  the  ventral  pancreas  so  that  it  opens  dorsally  and  somewhat  to  the  left  into  the 
bile  duct.  At  the  same  time,  the  ventral  pancreas  is  brought  into  close  proximity 
to  the  dorsal  pancreas,  and  the  duct  of  the  latter  is  shifted  to  the  left  side  of  the 
intestine  (Figs.  178  and  180).     It  is  also  carried  further  cephalad  during  the  course 


Accessory  pancreatic  duct 
Dorsal  pancreas 


Ventral  pancreas 
Pancreatic  duct 
Bile  duct 


Accessory  pancreatic  duct 
Dorsal  pancreas 


I        I     Ventral  pancreas 
Bile  duct    Fancreatic  duct 


Fig.  180. — Two  stages  showing  the  development  of  the  pancreas.    A,  at  live  weeks;  /->,  at  seven  weeks 

(after  Kollman). 


of  development  so  that  in  the  adult  the  interval  between  the  ducts  is  from  10  to 

35  mm- 

In  embryos  of  20  mm.  the  tubules  of  the  dorsal  and  ventral  pancreatic  an- 
lages interlock  (Fig.  180  B).  Eventually,  anastomosis  takes  place  between  the 
two  ducts  and  the  duct  of  the  ventral  pancreas  persists  as  the  functional  pan- 
creatic duct  of  the  adult.  The  proximal  portion  of  the  dorsal  pancreatic  duct 
forms  the  accessory  duct  which  remains  pervious,  but  becomes  a  tributary  of 
the  ventral  pancreatic  duct.  The  ventral  pancreas  forms  part  of  the  head 
and  uncinate  process  of  the  adult  gland.  The  dorsal  pancreas  takes  part  in 
forming  the  head  and  uncinate  process  and  comprises  the  whole  of  the  body  and 
tail. 

In  10  mm.  embryos  the  portal  vein  separates  the  two  pancreatic  anlages  and  later  they 
partially  surround  the  vein.  The  chief  branch  of  the  portal  in  the  adult,  the  superior  mesenteric 
vein,  thus  passes  through  the  pancreas,  receiving  the  splenic  vein  which  courses  along  and  drains 


i88 


THE   ENTODERMAL   CANAL  AND   ITS   DERIVATIVES 


the  dorsal  surface  of  the  tail.  The  alveoli  of  the  gland  are  developed  as  darkly  staining  cellular 
buds  in  embryos  of  40  to  55  mm.  The  islands  characteristic  of  the  pancreas  appear  first  in  the 
tail  at  55  mm. 

Owing  to  the  shift  in  the  position  of  the  stomach  and  duodenum  during  development 
the  pancreas  takes  up  a  transverse  position,  its  tail  extending  to  the  left.  To  its  ventral  sur- 
face is  attached  the  transverse  mesocolon. 


BODY  CAVITIES,  DIAPHRAGM  AND  MESENTERIES 
The  Primitive  Ccelom  and  Mesenteries. — In  the  Peters  embryo  the  primary 
mesoderm  has  already  split  to  form  the  extra-embryonic  ccelom  (Fig.   232). 
When  the  intra-embryonic  mesoderm  differentiates,  numerous  clefts  appear  on 

either  side  between  the  somatic  and  splanchnic 
layers  of  mesoderm.  These  clefts  coalesce  in 
the  cardiac  region  and  form  two  elongated  cavi- 
ties lateral  to  the  paired  tubular  heart.  Simi- 
larly, right  and  left  pleuro-peritoneal  cavities  are 
formed  between  the  mesoderm  layers  caudal  to 
the  heart.  The  paired  pericardial  cavities  ex- 
tend toward  the  midline  cranially  and  com- 
municate with  each  other  (Fig.  181).  They 
also  are  prolonged  caudally  until  they  open 
into  the  pleuro-peritoneal  cavities.  These  in 
turn  communicate  laterally  with  the  extra- 
embryonic ccelom.  In  an  embryo  of  1.5  mm. 
the  ccelom  thus  consists  of  a  U-shaped  peri- 
cardial cavity,  the  right  and  left  limbs  of  which 
are  continued  caudally  into  the  paired  pleuro-peritoneal  cavities ;  these  extend 
out  into  the  extraembryonic  ccelom. 

When  the  head-fold  and  fore-gut  of  the  embryo  are  developed,  the  layers  of 
splanchnic  mesoderm  containing  the  heart  tubes  are  folded  together  ventral  to 
the  fore-gut  and  form  the  ventral  mesentery  between  the  gut  and  the  ventral  body 
wall  (Fig.  182).  Owing  to  the  position  of  the  yolk-sac,  the  caudal  extent  of 
the  ventral  mesentery  is  limited.  At  the  level  on  each  side,  where  the  vitello- 
umbilical  trunk  courses  to  the  heart,  the  splanchnic  mesoderm  and  the  somatic 
mesoderm  are  united  (Fig.  182).  Thus  is  formed  the  septum  transversum,  which 
separates  the  ventral  mesentery  into  a  cranial  and  caudal  portion.  Cranial  to 
the  septum,  the  heart  is  suspended  in  the  ventral  mesentery  which  forms  the 
dorsal  and  ventral  mesocardia  (Fig.  183  A).     Into  the  ventral  mesentery  caudal 


Pericardial  Cauitu 

Surface  of 
fore-gut 


Pleuro- pericardia  I 
Canal 

Entoderm  of  out 


Peri  tonea  I  Cavity 

xtra-embryonic 
coelom 

-Wall  of  yolk-sac 

Fig.  181. — Diagrammatic  dor- 
sal view  of  the  coelom  in  an  early 
human  embryo  (modified  after  Rob- 
inson). 


BODY    CAVITIES,    DIAPHRAGM    AND    MESENTERIES 


189 


to  the  septum  grows  the  liver.     This  portion  of  the  ventral  mesentery  gives  rise 
dorsally  to  the  lesser  omentum  of  the  stomach  and,  with  the  septum  transversum, 


Esophagus 


Spinal  CoroL 


Pericardial 
Cavity 

Ventricle  of 
heart 


Ventral  mesocardium 

Liver 
ventral  mesentery 
(falciform  I^LVS^U 


Dorsal 

me  so  card- 
i u m 


Septum 

transversum 

Stomach 

Ventral  mesentery 

(lesser  omentum) 

Dorsal 

mesogastrium 

Dorsal 

pancreas 


tei 


Mesocolon 


'  Mesorectum 


FlG.  182. — Diagram  showing  the  primitive  mesenteries  of  an  early  human  embryo  in  median  sagittal  sec- 
tion.    The  broken  lines  A,  B,  and  C  indicate  the  level  of  sections  A,  B,  and  C  in  Fig.  183. 


neural  tube 


Neural iube 


Notochord 

Aorta.  A/otochord 

Post  cardinal     Aorta. 
Vein 

Dorsal  mesentery 
JJuodenum 

Lesser 
Om.».u»,         Guf 

Liver 

Peritoneal  Cavity 

Talciform 
liqament 


Fig.  183. — Diagrammatic  transverse  sections.     A ,  through  the  heart  and  pericardial  cavities  of  an  early 
human  embryo;  B,  through  the  stomach  and  liver;  C,  through  the  intestine  and  peritoneal  cavity. 


it  forms  the  ligaments  of  the  liver.     Ventrally  it  persists  as  the  falciform  ligament 
(Fig.  183  B). 


190 


THE    ENTODERMAL    CANAL   AND    ITS    DERIVATIVES 


Dorsal  to  the  gut  the  splanchnic  mesoderm  of  ea*ch  side  is  folded  together  in 
the  median  sagittal  plane  and  constitutes  the  dorsal  mesentery  which  extends  to 
the  caudal  end  of  the  digestive  canal  (Figs.  182  and  183  C).  This  suspends  the 
stomach  and  intestine  from  the  dorsal  body  wall  and  is  divided  into  the  dorsal 
mesogastrium  or  greater  omentum  of  the  stomach ;  the  mesoduodenum,  the  mesen- 
tery proper  of  the  small  intestine,  the  mesocolon  and  the  mesorectum. 

The  covering  layers  of  the  viscera,  of  the  mesenteries  and  of  the  body  wall, 
are  continuous  with  each  other  and  consist  of  a  mesothelium  overlying  connective 
tissue.     They  are  derived  from  the  somatic  and  splanchnic  layers  of  mesoderm. 


Bulbus  cordis         Dorsal  mesocardium 


Pericardial  cavity 

Somatopleure 

Septum  transversum 

Liver  trabecules 
He  pic  diverticulum 

Yolk-stalk 


Sinus  venosus 

Lateral  mesocardium 
Common  cardinal  vein 

Umbilical  veiiir 

Vitelline  vein  overlying 
stomach 

Pleuro-peritoneal  canal 


Peritoneal  cavity 


Fig.  184. — Reconstruction  near  median  sagittal  plane    of  a  3  mm.  human  embryo,  showing  the  body 
cavities  and  septum  transversum  (Kollmann's  Handatlas). 

The  primitive  ccelom  lies  approximately  in  one  plane,  as  in  Fig.  181.  With 
the  development  of  the  head-fold  and  the  ventral  flexion  and  fusion  of  the  heart 
tubes,  the  pericardial  cavity  is  bent  ventrad  and  enlarged.  The  ventral  meso- 
cardium attaching  the  heart  to  the  ventral  body  wall  disappears  and  the  right 
and  left  limbs  of  the  U-shaped  cavity  become  confluent  ventral  to  the  heart. 
The  result  is  a  single  large  pericardial  chamber,  the  long  axis  of  which  now  lies 
in  a  dorso-ventral  plane  nearly  at  right  angles  to  the  plane  of  the  pleuro-peri- 
toneal  cavities,  and  connected  with  them  dorsally  by  the  right  and  left  pleuro- 
peritoneal  canals  (Fig.  184). 

The  division  of  the  primitive  cazlom  into  separate  cavities  is  accomplished  by 


BODY   CAVITIES,    DIAPHRAGM   AND   MESENTERIES 


I9I 


the  development  of  three  membranes:  (1)  the  septum  transversum,  which  sepa- 
rates incompletely  the  pericardial  and  pleural  cavities  from  the  peritoneal  cavi- 
ties; (2)  the  pleuro- pericardial  membrane  which  completes  the  division  between 
pericardium  and  pleural  cavity;  (3)  the  pleuro-peritonenl  membrane  which  com- 
pletes the  partition  between  each  pleural  cavity  containing  the  lung  and  the 
peritoneal  cavity  which  contains  the 
abdominal  viscera. 

The  Septum  Transversum. — In  em- 
bryos of  2  to  3  mm.  (Fig.  184)  the 
splanchnic  mesoderm  of  the  yolk-sac 
and  that  of  the  heart  are  continuous 
where  the  vitelline  veins  cross  from  one 
layer  to  the  other;  also  where  the  um- 
bilical veins  course  from  the  body  wall 
to  the  heart  the  somatic  and  splanchnic 
layers  of  mesoderm  are  continuous. 
Thus,  there  is  formed  caudal  to  the 
heart  a  transverse  partition  filling  the 
space  between  the  sinus  venosus  of  the 
heart,  the  gut  and  the  ventral  body  wall 
and  separating  the  pericardial  and  peri- 
toneal cavities  from  each  other  ventral 
to  the  gut.  This  mesodermal  partition 
was  termed  by  His  the  septum  trans- 
vcrsum. It  is  the  anlage  of  a  large  part 
of  the  diaphragm.  At  first  it  does  not 
extend  dorsal  to  the  gut,  but  leaves 
on  either  side  a  pleuro-peritoneal  canal 
through  which  the  pericardial  and 
pleuro-peritoneal  cavities  communicate 

(Fig.  183).  In  embryos  of  4  to  5  mm.  the  lungs  develop  in  the  median  walls  of 
these  canals  and  bulge  laterally  into  them.  Thus  the  canals  become  the  pleural 
cavities  and  will  be  so  termed  hereafter. 

On  account  of  the  more  rapid  growth  of  the  embryo,  there  is  an  apparent 
constriction  at  the  yolk-stalk  and,  with  the  development  of  the  umbilical  cord, 
the  peritoneal  cavity  is  finally  separated  from  the  extra-embryonic  ccelom. 


5  4   3 
L 

Fig.  185. — Diagram  showing  the  change 
in  position  of  the  septum  transversum  in  stages 
from  2  to  24  mm.  (modified  after  Mall).  The 
septum  is  indicated  at  different  stages  by  the 
numerals  to  the  left,  the  numbers  correspond- 
ing to  the  length  of  the  embryo  at  each  stage. 
The  letters  and  numbers  at  the  right  represent 
the  segments  of  the  occipital,  cervical,  thoracic 
and  lumbar  regions. 


192 


THE    ENTODERMAL    CANAL   AND    ITS   DERIVATIVES 


Dorsally  the  pleural  and  peritoneal  cavities  are  permanently  partitioned  length- 
wise by  the  dorsal  mesentery. 

The  septum  transversum  in  2  mm.  embryos  occupies  a  transverse  position  in 
the  middle  cervical  region  (Fig.  185,  2).  According  to  Mall,  it  migrates  caudally, 
its  ventral  portion  at  first  moving  more  rapidly  so  that  its  position  becomes 
oblique.  In  5  mm.  embryos  (Fig.  185,  5)  it  is  opposite  the  fifth  cervical  segment, 
at  which  level  it  receives  the  phrenic  nerve.     In  stages  later  than  7  mm.  the  sep- 


Pen 'cardial 
ccu/ity 


Com.  cardinal  vein 


Pleuro  -pencardia.L 
membrane. 


Vleuro- 
peritoneal 
membrane 


Ye  in  to 
limb  bud 


Stomach 


Mesonephros 


Fig.  186. — Reconstruction  of  a  7  mm.  embryo  showing  from  the  left  side  the  pleuro-pericardial  mem- 
brane, the  pleuro-peritoneal  membrane  and  the  septum  transversum  (after  Mall). 


turn  migrates  caudad,  until  at  24  mm.  it  is  opposite  the  first  lumbar  segment. 
During  this  second  period  of  migration  its  dorsal  attachment  travels  faster  than 
its  ventral  portion.  Therefore,  it  rotates  to  a  position  nearly  at  right  angles  to 
its  plane  in  7  mm.  embryos  and  its  original  dorsal  surface  becomes  its  ventral 
surface.  In  connection  with  the  septum  transversum  two  other  membranes 
develop. 

The  Pleuro-pericardial  and  Pleuro-peritoneal  Membranes. — The  common 


BODY   CAVITIES,    DIAPHRAGM   AND   MESENTERIES 


193 


cardinal  veins  (ducts  of  Cuvier)  on  their  way  to  the  heart  curve  around  the  pleural 
cavities  laterally  in  the  body  wall  (Figs.  184  and  186).  In  embryos  of  7  mm. 
each  vein  forms  a  ridge  which  projects  from  the  body  wall  mesially  into  the  pleu- 
ral canals.  This  ridge,  the  pulmonary  ridge  of  Mall,  later  broadens  and  thickens 
cranio-caudally  (Fig.  186).  Its  cranial  and  caudal  margins  form  two  sides  of  a 
spherical  triangle,  the  third  side  or  base  of  which  is  the  line  of  attachment  of  the 
dorsal  mesentery  to  the  body  wall  (Fig.  187).     At  its  ventral  angle  the  sides  of 


Phuropericard.ia.1  membra-ne. 
Phrenic  nerve 


Pericardial  cavity 


iiusn  Iratisversum 


Pleuro- 

peritonea.1 

Membrane 


Mesonep/iros 


fomach 


Fig.  187. — Reconstruction  of  an  11  mm.  embryo  to  show  the  same  structures  as  in  Fig.  186  (after  Mall). 


this  triangle  are  continuous  with  the  septum  transversum.  Its  cranial  side  forms 
the  pleuro-pericardial  membrane  and  in  9  to  10  mm.  embryos  reduces  the  opening 
between  the  pleural  and  pericardial  cavities  to  a  mere  slit.  Its  caudal  side  be- 
comes the  pleuro-peritoneal  membrane,  which  eventually  separates  dorsally  the 
pleural  from  the  peritoneal  cavity.  The  membranes  at  first  lie  nearly  in  the  sagit- 
tal plane  and  a  portion  of  the  lung  is  caudal  to  the  pleuro-peritoneal  membranes 
(Fig.  186).  Between  the  stages  of  7  and  n  mm.  the  dorsal  attachment  of  the 
13 


194 


THE    ENTODEKMAL    CANAL   AND    ITS    DERIVATIVES 


septum  transversum  is  carried  caudally  more  rapidly  than  its  ventral  portion 
and  its  ventral  surface  becomes  its  dorsal  side  (Figs.  186  and  187).  The  pleuro- 
peritoneal  membrane  is  carried  caudad  with  the  septum  transversum  until  the 
lung  lies  in  the  angle  between  the  pleuro-peritoneal  and  pleuro-pericardial  mem- 
branes and  is  included  within  the  spherical  triangle  which  has  been  described 
above  (Fig.  187).  The  dorsal  end  of  the  pleuro-pericardial  membrane  lags  behind 
and  so  takes  up  a  position  in  a  coronal  plane  nearly  at  right  angles  to  the  septum 
transversum  (Figs.  187  and  188).  In  n  mm.  embryos  the  pleuro-pericardial 
membranes  have  fused  completely  on  each  side  with  the  median  walls  of  the  pleural 


Pleural 
cavity 


Mesoderm  of  left  luntjlucL 
Pericardial  cavity 


PI  euro  -peritoneal 
membrane 
Phrenic  nerve 


Wall  of 
heart 


.iver 


falciform 
lioament 


Septum  transversum 


Fig.  188. — Transverse  section  through  a  10  mm.  human  embryo  showing  the  pleuro-pericardial  mem- 
brane separating  the  pericardium  from  the  pleural  cavities.     X  33. 


canals  and  thus  separate  the  pericardium  from  the  paired  pleural  cavities.  By 
way  of  the  pleuro-pericardial  membranes  the  phrenic  nerves  course  to  the  septum 
transversum  (Fig.  187). 

The  pleuro-peritoneal  membranes  are  continued  dorsally  and  caudally  along 
the  mesonephric  folds;  ventrally  and  caudally  they  become  on  the  liver  the 
dorsal  pillars  of  the  diaphragm  or  coronary  appendages  (Lewis)  (Fig.  189).  Be- 
tween the  free  margins  of  the  membranes  and  the  mesentery  an  opening  is  left 
on  each  side,  through  which  the  pleural  and  peritoneal  cavities  communicate 
(Figs.  187  and  193). 


BODY    CAVITIES,    DIAPHRAGM    AND    MESENTERIES 


195 


Owing  to  the  caudal  migration  of  the  septum  transversum  and  the  growth  of 
the  lungs  and  liver,  the  pleuro-peritoneal  membrane,  at  first  lying  in  a  nearly 
sagittal  plane,  is  shifted  to  a  horizontal  position  and  gradually  its  free  margin 
unites  with  the  dorsal  pillars  of  the  diaphragm  and  with  the  dorsal  mesentery. 
The  opening  between  the  pleural  and  peritoneal  cavities  is  thus  narrowed  and 
finally  closed  in  embryos  of  19  to  20  mm. 

The  Diaphragm  and  Pericardial  Membrane. — The  lungs  grow  and  expand, 
not  only  cranially  and  caudally,  but  also  laterally  and  ventrally  (Fig.  190  A,  B). 
Room  is  made  for  them  by  the  obliteration  of  the  very  loose,  spongy  mesenchyme 


Coronary 

appendaq 
of  liver 

Venacaya 
inferior 


Pleural  Cavity 


Pleuro-peritoneal 
membrane 

^fffi — Phrenic  nerve  in 

septum  transversum 


Fig.  189. — Transverse  section  through  a  10  mm.  embryo  showing  the  pleuro-peritoneal  membranes, 
X  16  (from  an  embryo  loaned  by  Dr.  H.  C.  Tracy). 


of  the  body  wall  (Fig.  189).  As  the  lungs  grow  laterally  and  ventrally  in  the 
body  wall  around  the  pericardial  cavity,  they  split  off  from  the  body  wrall  the 
pericardial  membrane  and  more  and  more  the  heart  comes  to  lies  in  a  mesial 
position  between  the  lungs  (Fig.  190  B).  The  pleural  cavities  thus  increase 
rapidly  in  size.  At  the  same  time,  the  liver  grows  enormously  and  on  either  side 
a  portion  of  the  body  wall  is  taken  up  into  the  septum  transversum  and  pleuro- 
peritoneal  membranes.  The  diaphragm,  according  to  Broman,  is  thus  derived 
from  four  sources  (Fig.  191):  (1)  its  ventral  pericardial  portion  from  the  septum 
transversum;  its  lateral  portions  from  (2)  the  pleuro-peritoneal  membranes  plus 
(3)  derivatives  from  the  body  wall;    lastly,  a  median  dorsal  portion  is  formed 


196 


THE   ENTODERMAL   CANAL  AND   ITS   DERIVATIVES 


from  (4)  the  dorsal  mesentery.  In  addition  to  these,  the  striated  muscle  of  the 
diaphragm,  according  to  Bardeen,  takes  its  origin  from  a  pair  of  pre-muscle 
masses  which  in  9  mm.  embryos  lie  one  on  each  side  opposite  the  fifth  cervical 
segment.     This  is  the  level  at  which  the  phrenic  nerve  enters  the  septum  trans- 


A 


Esophagus 


Septum  trans  v  ers  i 


Pleura- pericardial  canal 
,  Luna 


Peri  ea.rdt.al  cavity 


'Pleura-peritoneal  membrane 


Pleural  cavity 
Pericardial 
Heart       membrane 


Fig.  190. — Diagrams  showing  the  development  of  the  lungs  and  the  formation  of  the  pericardial  mem- 
brane (modified  after  Robinson).    A,  coronal  section;  B,  transverse  section. 


Fig.  191. — Diagram  showing  the  origin  of  the  diaphragm  (after  Broman).  1,  septum  transver- 
sum;  2,  3,  derivatives  of  mesentery;  4,  4,  derivatives  of  pleuro-peritoneal  membrane;  5,  5,  parts  de- 
rived from  the  body  walls. 

versum.  The  exact  origin  of  these  muscle  masses  is  in  doubt  but  they  probably 
represent  portions  of  the  cervical  myotomes  of  this  region.  The  muscle  masses 
migrate  caudally  with  the  septum  transversum  and  develop  chiefly  in  the  dorsal 
portion  of  the  diaphragm,  according  to  Bardeen. 

Keith  derives  the  muscle  of  the  diaphragm  also  from  the  rectus  and  transversalis  muscles 
of  the  abdominal  wall. 

The  cavities  of  the  mesodermic  segments  are  regarded  as  portions  of  the  ccclom  but  in 


BODY    CAVITIES,    DIAPHRAGM    AXD    MESENTERIES 


197 


man  they  disappear  early.     The  development  of  the  vaginal  sacs  which  grow  out  from  the 
inguinal  region  of  the  peritoneal  cavity  into  the  scrotum  will  be  described  in  Chapter  VIII. 

The  Omental  Bursa  or  Lesser  Peritoneal  Sac. — According  to  Broman,  the 
lesser  peritoneal  sue  is  represented  in  3  mm.  embryos  by  a  peritoneal  pocket  which 
extends  craniallv  into  the  dorsal  mesentery  to  the  right  of  the  esophagus.  A 
similar  pocket  present  on  the  left  side  has  disappeared  in  4  mm.  embryos.  Lateral 
to  the  opening-of  the  primitive  peritoneal  sac,  a  lip-like  fold  of  the  mesentery  is 
continued  caudally  along  the  dorsal  body  wall  into  the  mesonephric  fold  as  the 


Fight 'umbilical  vein 
Ventral  mesentery 


flight  lobe  I    > 
of  Liver 


Lesser 
peritoneal 


Left  umbilical  Vein 


Ectoderm  of 
body  wall 


Left,  lobe 
of  Liver 

Ventral 
mesentery 

Duodenum 


Dorsal 
mesentery 

Left  post , 
Cardinal  vein 


Notochord 


Fig.  192. — Diagrammatic  view  of  an  embryo  of  7  to  9  mm.  showing  the  position  of  the  lesser  peri- 
toneal sac.  The  cranial  portion  of  the  embryo  is  represented  as  sectioned  transversely,  caudal  to  the 
liver,  so  that  one  looks  at  the  caudal  surface  of  the  section  and  of  the  liver  and  craniallv  into  the  body 
cavities. 


plica  vena:  cava;,  in  which  later  the  inferior  vena  cava  develops  (Fig.  192).  The 
liver,  it  will  be  remembered,  grows  out  into  the  ventral  mesentery  from  the  fore- 
gut  and,  expanding  laterally  and  ventrally,  takes  the  form  of  a  crescent.  Its 
right  lobe  comes  into  relation  with  the  plica  venae  cavae  and,  growing  rapidly 
caudad,  forms  with  the  plica  a  partition  between  the  lesser  sac  and  the  peritoneal 
cavity.  Thus  the  cavity  of  the  lesser  peritoneal  sac  is  extended  caudally  from 
a  point  opposite  the  bifurcation  of  the  lungs  to  the  level  of  the  pyloric  end  of  the 
stomach.  In  5  to  10  mm.  embryos  it  is  crescent-shaped  in  cross-section  (Fig. 
132)  and  is  bounded  mesially  by  the  greater  omentum  (dorsal  mesentery)  and  the 
right  wall  of  the  stomach,  laterally  by  the  liver  and  plica  venae  cavae  and  ven- 


198 


THE   ENTODERMAL   CANAL   AND    ITS    DERIVATIVES 


trally  by  the  lesser  omentum  (ventral  mesentery).  It  communicates  to  the 
right  with  the  peritoneal  cavity  through  an  opening  between  the  liver  ventrally 
and  the  plica  venae  cavae  dorsally  (Fig.  194).  This  opening  is  the  epiploic  fora- 
men (of  Winslow).  When  the  dorsal  wall  of  the  stomach  rotates  to  the  left  the 
greater  omentum  is  carried  with  it  to  the  left  of  its  dorsal  attachment.     Its  tissue 


Body  wall 


Inf.  Vena  cava. 


Sup.  recess 
of  lesser 
peritonea]  ^ac  E 

Pleuro -peritoneal' 
tnetnbrane 

Inf.vena. 
cava. 

~Phca  venae 
Cai/ae 


Mesonephric 
fold 

Genital 
fold 


ra/fciform  ligament 


Coronary 
attachment  of 
Liver  to  diaphragm 


~Pleu.ro  - 
peritoneal 

Pleuro- 
peritoneal 
membrane 
Lesser 
Omentum 

Greater 
omentum 

Spleen 
Stomach 


Lesser 

peritoneal  3ac 
orTa. 


Fig.  193. — A  diagrammatic  ventral  view  of  the  middle  third  of  an  embryo  12  to  15  mm.  long.  The 
figure  shows  the  caudal  surface  of  a  section  through  the  stomach  and  spleen;  a  ventral  view  of  the  stom- 
ach, the  liver  having  been  cut  away  to  leave  the  sectioned  edges  of  the  lesser  omentum  and  plica  venae 
cava;;  and  the  caudal  surface  of  the  septum  transversum  and  pleuro-peritoneal  membrane.  Upon  the 
surface  of  the  septum  is  indicated  diagrammatically  the  attachment  of  the  liver  (based  on  figures  of  Mall 
and  F.  T.  Lewis  and  model  by  H.  C.  Tracy). 


grows  actively  to  the  left  and  caudally  and  gives  the  omentum  an  appearance  of 
being  folded  on  itself  between  the  stomach  and  the  dorsal  body  wall  (Fig.  193). 
The  cavity  of  the  lesser  peritoneal  sac  is  carried  out  between  the  folds  of  the 
greater  omentum  as  the  inferior  recess  of  the  omental  bursa. 

From  the  cranial  end  of  the  sac  there  is  constricted  off  a  small  closed  cavity  which  is 
frequently  persistent  in  the  adult.     This  is  the  bursa  infracardiaca  and  may  be  regarded  as  a 


BODY    CAVITIES,    DIAI'IIKACM    AM)    MESENTERIES 


IQ9 


third  pleural  cavity.     It  lies  to  the  right  of  the  esophagus  in  the  mediastinum  and  its  average 
diameter  in  the  adult  is  10  mm. 

When  the  stomach  changes  its  position  and  form  so  that  its  mid-ventral 
line  becomes  the  lesser  curvature  and  lies  to  the  right,  the  position  of  the  lesser 
omentum  is  also  shifted.  From  its  primitive  location  in  a  median  sagittal  plane 
with  its  free  edge  directed  caudally  it  is  rotated  through  900  until  it  lies  in  a  cor- 
onal plane  with  its  free  margin  facing  to  the  right.     The  epiploic  foramen  now 


Mesonephros 
Greater  omentum 


peritoneal 
sac 

Duo- 


Vitilline 
vein 

Intestinal- 
loop 


Stomach 


Left  umbilical  Vein 


Fig.  194. — An  obliquely  transverse  section  through  a  10  mm.  embryo  at  the  level  of  the  epiploic  foramen 

(of  Winslow).     X  S3- 


forms  a  slit-like  opening  leading  from  the  peritoneal  cavity  into  the  vestibule 
of  the  omental  bursa.  The  foramen  is  bounded  ventrally  by  the  edge  of  the 
lesser  omentum,  dorsally  by  the  inferior  vena  cava,  cranially  by  the  caudate  pro- 
cess of  the  liver  and  caudally  by  the  wall  of  the  duodenum. 

During  fetal  life  the  greater  omentum  grows  rapidly  to  the  left  and  caudad 
in  the  form  of  a  sac  flattened  dorso-ventrally.  It  overlies  the  intestines  ven- 
trally and  contains  the  inferior  recess  of  the  omental  bursa  (Fig.  195).  The  dor- 
sal wall  of  the  sac  during  the  third  and  fourth  months  usually  fuses  with  the  trans- 
verse colon  where  it  overlies  the  latter.     Caudal  to  this  attachment,  the  walls 


200 


THE    ENTODERMAL    CANAL   AND    ITS    DERIVATIVES 


of  the  greater  omentum  may  be  fused  and  its  cavity  is  then  obliterated.  The 
inferior  recess  of  the  omental  bursa  thus  may  be  limited  in  the  adult  chiefly  to  a 
space  between  the  stomach  and  the  dorsal  fold  of  the  greater  omentum,  which 
latter  is  largely  fused  to  the  peritoneum  of  the  dorsal  body  wall.  The  spleen 
develops  in  the  cranial  portion  of  the  greater  omentum  and  that  portion  of  the 
omentum  which  extends  between  the  stomach  and  spleen  is  known  as  the  gastro- 
lienic  ligament.  The  dorsal  wall  of  the  omentum  between  the  spleen  and  kidney 
is  the  lieno-renal  ligament. 

Further  Differentiation  of  the  Mesenteries:  Ligaments  of  the  Liver. — We 
have  seen  (p.  188)  that  the  cranial  portion  of  the  ventral  mesentery  forms  the 
mesocardium  of  the  heart.     In  the  ventral  mesentery  caudal  to  the  septum  trans- 


A  B  c 

Fig.  195. — Diagrams  showing  the  development  of  the  mesenteries  (Hertwig).  A,  illustrates  the 
beginning  of  the  great  omentum  and  its  independence  of  the  transverse  mesocolon;  in  B  the  two  come 
into  contact;  in  C  they  have  fused;  A,  stomach;  B,  transverse  colon;  C,  small  intestine;  D,  duode- 
num; E,  pancreas;   F,  greater  omentum. 


versum  develops  the  liver.  From  the  first,  it  is  enveloped  in  folds  of  the  splanch- 
nic mesoderm  which  give  rise  to  its  capsule  and  ligaments  as  the  liver  increases 
in  size  (Fig.  183  B).  Wherever  the  liver  is  unattached,  the  mesodermal  layers 
of  the  ventral  mesentery  form  its  capsule  (of  Glisson),  a  fibrous  layer  covered 
by  mesothelium  continuous  with  that  of  the  peritoneum  (Fig.  183  B).  Along 
its  mid-dorsal  and  mid- ventral  line  the  liver  remains  attached  to  the  ventral 
mesentery.  The  dorsal  attachment  between  the  liver,  stomach  and  duodenum 
is  the  lesser  omentum.  This  in  the  adult  is  differentiated  into  the  duodeno-hepatic 
and  gastro-hepatic  ligaments.  The  attachment  of  the  liver  to  the  ventral  body 
wall  extends  caudally  to  the  umbilicus  and  forms  the  falciform  ligament. 

In  its  early  development  the  liver  abuts  upon  the  septum  transversum,  and 
in  4  to  5  mm.  embryos  is  attached  to  it  along  its  cephalic  and  ventral  surfaces. 


BODY    CAVITIES,    DIAPHK.U.M    AND    MESENTERIES  201 

Soon  dorsal  prolongations  of  the  lateral  liver  lobes,  the  coronary  appendages 
come  into  relation  with  the  septum  dorsally  and  laterally.  The  attachment  of 
the  liver  to  the  septum  transversum  now  has  the  form  of  a  crescent,  the  dorsal 
horns  of  which  arc  the  coronary  appendages  (Fig.  193).  This  attachment  be- 
comes the  coronary  ligament  of  the  adult  liver.  The  dorso- ventral  extent  of  the 
coronary  ligament  is  reduced  during  development  and  its  lateral  extensions  upon 
the  diaphragm  give  rise  to  the  triangular  ligaments  of  each  side. 

The  right  lobe  of  the  liver,  as  we  have  seen,  comes  into  relation  along  its 
dorsal  surface  with  the  plica  voice  cava:  in  9  mm.  embryos  (Figs.  192  and  193). 
This  attachment  extends  the  coronary  ligament  caudally  on  the  right  side  and 
makes  possible  the  connection  between  the  veins  of  the  liver  and  mesonephros 
through  which  the  inferior  vena  cava  is  in  part  developed.  The  portion  of  the 
liver  included  between  the  plica  vena  cavce  and  the  lesser  omentum  is  the  cau- 
date lobe  (of  Spigelius). 

In  a  fetus  of  five  months  the  triangular  ligaments  mark  the  position  of  the 
lateral  coronary  appendages.  The  umbilical  vein  courses  in  a  deep  groove  along 
the  ventral  surface  of  the  liver  and  with  the  vena  porta  and  gall-bladder  bounds 
the  quadrate  lobe. 

Changes  in  tlie  Dorsal  Mesentery. — That  part  of  the  digestive  canal  which 
lies  within  the  peritoneal  cavity  is  suspended  by  the  dorsal  mesentery  which  at  first 
forms  a  simple  attachment  extending  in  the  median  sagittal  plane  between  body 
wall  and  primitive  gut.  That  portion  of  it  connected  with  the  stomach  forms 
the  greater  omentum,  the  differentiation  of  which  has  been  described  (p.  199). 
The  mesentery  of  the  intestine  is  carried  out  into  the  umbilical  cord  between  the 
limbs  of  the  intestinal  loop.  When  the  intestine  elongates  and  its  loop  rotates, 
the  caecal  end  of  the  large  intestine  comes  to  lie  cranially  and  to  the  left,  the  small 
intestine  caudally  and  to  the  right,  the  future  duodenum  and  colon  crossing  in 
close  proximity  to  each  other  (Fig.  196  A).  On  the  return  of  the  intestinal  loop 
into  the  abdomen  from  the  umbilical  cord  the  caecal  end  of  the  colon  lies  to  the 
right  and  the  transverse  colon  crosses  the  duodenum  ventrally  and  cranially. 
The  primary  loops  of  the  small  intestine  lie  caudal  and  to  the  left  of  the  ascending 
colon  (Fig.  196  B).  There  has  thus  been  a  torsion  of  the  mesentery  about  the  base 
of  the  superior  mesenteric  artery  as  an  axis.  From  this  focal  point  the  mesen- 
tery of  the  small  intestine  and  colon  spreads  out  fan-like.  The  mesoduodenum  is 
pressed  against  the  dorsal  body  wall,  fuses  with  its  peritoneal  layer  and  is  obliter- 
ated (Fig.  195).  Where  the  mesentery  of  the  transverse  colon  crosses  the  duo- 
denum it  fuses  at  its  base  with  the  surface  of  the  latter  and  of  the  pancreas.     Its 


202 


THE   ENTODERMAL   CANAL  AND    ITS   DERIVATIVES 


fixed  position  now  being  transverse  instead  of  sagittal,  the  mesentery  is  known  as 
the  transverse  mesocolon.  The  mesentery  of  the  ascending  colon  is  flattened 
against  the  dorsal  body  wall  on  the  right  and  fuses  with  the  peritoneum.  Simi- 
larly, the  descending  mesocolon  fuses  to  the  body  wall  of  the  left  side  (Fig.  196 
A,  B).  There  are  thus  left  free  (1)  the  transverse  mesocolon;  (2)  the  mesentery 
proper  of  the  jejunum  and  ileum  with  numerous  folds  corresponding  to  the  loops 
of  the  intestine;  (3)  the  iliac  mesocolon;  (4)  the  mesorectum,  which  retains  its 
primitive  relations. 

Anomalies  of  the  diaphragm  and  mesenteries  are  not  uncommon.     The  per- 


Lesser  omentum 


Dorsal  . 
mesogastrLLLm 


Stomach 

Greater 
omentum 

Transverse 
mesocolon 


ecum 


Mesentery 
Mesorectum 

Iliac   mesocolon 
A  B 

Fig.  196. — Diagrams  showing  the  development  of  the  mesenteries  in  ventral  view  (modified  from 
Tourneux  in  Heisler).  *  Cut  edge  of  greater  omentum;  a,  area  of  ascending  mesocolon  fused  to  dorsal 
body  wall;  b,  area  of  descending  mesocolon  fused  to  dorsal  body  wall. 

sistence  of  a  dorsal  opening  in  the  diaphragm,  more  commonly  on  the  left  side, 
may  be  explained  as  due  to  the  defective  development  of  the  pleuro-peritoneal 
membrane.  Such  a  defect  may  lead  to  diaphragmatic  hernia,  the  abdominal 
viscera  projecting  to  a  greater  or  less  extent  into  the  pleural  cavity. 

The  mesenteries  also  may  show  malformations  due  to  the  persistence  of  the 
simpler  embryonic  conditions,  usually  correlated  with  the  defective  development 
of  the  intestinal  canal.  The  ascending  and  descending  mesocolon  may  be  free, 
having  failed  to  fuse  with  the  dorsal  peritoneum.  The  primary  folds  of  the  greater 
omentum  may  fail  also  to  unite  so  that  the  inferior  recess  extends  to  the  caudal 
end  of  the  greater  omentum. 


CHAPTER  VIII 

UROGENITAL  SYSTEM 

The  urogenital  system  is  composed  of  distinct  urinary  and  genital  glands 
which,  however,  possess  common  ducts  and  have  a  common  origin  from  the  meso- 
derm. The  excretory  glands  are  the  pronephros,  mesoncphros,  and  metanepkros, 
organs  which  develop  in  this  order.  The  first  two  named  are  the  temporary  kid- 
neys of  the  mammalian  embryo,  but  are  functional  in  adult  fishes  and  amphibia. 
The  metanephros  is  the  permanent  kidney  of  reptiles,  birds  and  mammals. 

THE  PRONEPHROS 
The  pronephros,  when  functional,  consists  of  paired  segmentally  arranged 
tubules,  one  end  of  each  tubule  opening  into  the  ccelom,  the  other  into  a  longitu- 


*—*■  Antaqes  ef 
prgnephrie.  duct 


^  Notochord 

Fig.  197. — Diagrams  showing  the  development  of  the  pronephric  duct  and  pronephric  tubules  (modi- 
fied  from   Felix).     A    shows   a    later   stage    than   B. 

dinal  pronephric  duct  which  drains  into  the  cloaca  (Fig.   197  A).     Near  the 
nephrostome^the  opening  into  the  ccelom)  knots  of  arteries  project  into  the  tubules, 

203 


>o4 


UROGENITAL   SYSTEM 


forming  glomeruli.     Fluid  from  the  ccelom  and  glomeruli  and  excreta  from  the 
cells  of  the  tubules  are  carried  by  ciliary  movement  into  the  pronephric  ducts. 

The  human  pronephros  is  vestigial.  It  consists  of  seven  pairs  of  rudimentary 
pronephric  tubules  derived  from  the  mesoderm  of  the  nephrotomes  (Fig.  198), 
which  are  segmented  portions  of  the  cell  mass  intermediate  between  the  primitive 
segments  and  the  mesodermal  layers  (somatic  and  splanchnic).  Anlages  of 
pronephric  tubules  are  formed  as  dorsal  nodules  in  each  segment  from  the  seventh 
to  the  fourteenth.  The  nodules  hollow  out  and  open  into  the  ccelom.  Dorsally 
and  laterally,  the  tubules  of  each  side  unite  to  form  a  longitudinal  collecting  duct. 


Neural  tube 


Notochord 


Cavity  of  gut 


Splanchnic  mesoderm 


Mesoderm  of  yolk-sac 


« Mesodermal  segment 
Cavity  of  segment 

Intermediate  cell  mass 
Anlage  of  extremity 

Ccelom 

Y3  -yfliy.-Q.-  _ Somatic  mesoderm 


Umbilical  vein 


Fig.  198. — Transverse  section  of  a  2.4  mm.  human  embryo  showing  the  intermediate  cell  mass  or 
nephrotome   (Kollmann's  Atlas). 


The  tubules  first  formed  in  the  seventh  segment  begin  to  degenerate  before  those 
of  the  fourteenth  segment  have  developed.  Caudal  to  the  fourteenth  segment 
no  pronephric  tubules  are  developed,  but  the  free  end  of  the  collecting  duct  ap- 
parently grows  caudad,  beneath  the  ectoderm  and  lateral  to  the  nephrogemc  cord, 
until  it  reaches,  and  opens  through,  the  lateral  wall  of  the  cloaca.  Thus  are 
formed  the  paired  primary  excretory  {pronephric)  ducts.  The  pronephric  tubules 
begin  to  appear  in  embryos  of  1.7  mm.  (Felix  in  Keibel  and  Mall,  vol.  2);  in 
2.5  mm.  embryos  all  the  tubules  have  developed  and  the  primary  excretory  duct 
is  nearly  complete.     In  4.25  mm.  embryos  the  duct  has  reached  the  wall  of  the 


Tin:   mi.sum.i-iiros 


205 


cloaca  and  soon  after  fuses  with  it.  The  pronephric  tubules  soon  degenerate, 
but  the  primary  excretory  ducts  persist  and  become  the  ducts  of  the  mesoncphroi, 
or  mid-kidneys. 

THE  MESONEPHROS 

The  mesonephros,  like  the  pronephros,  consists  essentially  of  a  series  of 
tubules,  each  of  which  at  one  end  is  related  to  a  knot  of  blood-vessels  and  forms 
a.  capsule  surrounding  a  glomerulus,  at  the  other  end  opens  into  the  primary  ex-i)C*4< 
cretory  duct.     They  differ  from  the  pronephric  tubules  in  that  they  do  not  open  , 
into  the  ccelom,  and  as  many  as  four  may  develop  in  a  single  segment.     They     ~~ 
arise  from  the  mesoderm  intermediate  between  the  primitive  segments  and  the'-' 
lateral  mesodermal  layers,  mesoderm  which,  in  human  embryos,  is  not  segmented 
into  nephrotomes  caudal  to  the  tenth  pair  of  segments,  but  constitutes  the  un- 
segmented  nephrogenic  cord  on  either  side.     This  may  extend  caudally  as  far  as 
the  twenty-eighth  segment.     The  primary  excretory  ducts  lie  lateral  to  the  neph- 
rogenic cords.     When  the   mesonephric  tubules  begin  to  develop   and  expand 
there  is  not  room  for  them  in  the  dorsal  body  wall  and  as  a  result  this  bulges 
ventrally  into  the  ccelom.     Thus  there  is  produced  on  either  side  of  the  dorsal 
mesentery  a  longitudinal  urogenital  fold,  which  may  extend  from  the  sixth  cervi- 
cal  to  the  third  lumbar  segment  (Fig.  213).     Later,  this  ridge  is  divided  into  a 
lateral  mesonephric  fold  and  into  a  median  genital  fold,  the  anlage  of  the  genital 
gland. 

Differentiation  of  the  Tubules. — The  nephrogenic  cord  in  2.5  mm.  embryos 
first  divides  into  spherical  masses  of  cells,  the  anlages  of  the  mesonephric  tubules. 
Four  of  these  may  be  formed  in  a  single  segment.  Appearing  first  in  the  13th, 
14th  and  15th  segments,  the  anlages  of  the  tubules  differentiate  both  cranially 
and  caudally.  In  5.3  mm.  embryos  the  cephalic  limit  is  reached  in  the  sixth 
cervical  segment,  and  thereafter  degeneration  begins  at  the  cephalic  end.  In  7 
mm.  embryos  the  caudal  limit  is  reached  in  the  third  lumbar  segment  and  in 
later  stages  the  caudal  end  of  the  mesonephros  undergoes  degeneration. 

The  spherical  anlages  of  the  tubules  differentiate  in  a  cranio-caudal  direction 
(Fig.  199).  First,  vesicles  with  lumina  are  formed  (2.5  mm.).  Next  the  vesicles 
elongate  laterally,  unite  with  the  primary  excretory  ducts  and  become  S-shaped 
(4.9  mm.).  The  free  vesicular  end  of  the  tubule  enlarges,  becomes  thin-walled 
and  into  this  wall  grows  a  knot  of  arteries  to  form  the  glomerulus  (embryos  of 
5  to  7  mm.).  The  ^ubule,  at'first  solid,  hollows  out  and  is  lined  with  a  low  colum- 
nar epithelium-*.  The  outer  wall  of  the  vesicle  about  the  glomerulus  is  Bowman's 


2o6 


UROGENITAL   SYSTEM 


capsule,  the  two  constituting  a  renal  corpuscle  of  the  mesonephros  (Fig.  199  D). 
In  the  human  embryo,  the  tubules  do  not  branch  or  coil  as  in  pig  embryos,  con- 
sequently the  mesonephros  is  relatively  smaller.  At  10  mm.  32  to  34  tubules  are 
present  in  each  mesonephros  and  the  glomeruli  are  conspicuous  (Fig.  200). 
Each  tubule  shows  a  distal  excretory  and  a  proximal  collecting  portion  which 
connects  with  the  duct  (Fig.  201). 

The  glomeruli  form  a  single  median  column,  the  tubules  are  dorsal  and  the 


Meion*/jiriz    duct 


Anlaqe  of 
rnesonephric  tubule 


Tubule 


Bowmans  capsule 

Fig.  199. — Diagrams  showing  the  differentia- 
tion of  the  rnesonephric  tubules  (modified  after  Felix j . 
L,  lateral;  M,  median. 


Degenerating 

rnesonephric  corpuscieJ 


Degenerating 
Corpuscle  4  tubules 


'/refer  ^etaner*ns 


Fig.  200. — Diagram  showing  the  anlages 
of  the  urinary  organs  from  the  left  side  (based 
on  reconstructions  by  Keibel  and  Felix). 


Ventro-lateral  branches  from  the  aorta  supply  the 
glomeruli,  while  the  posterior  cardinal  veins,  dorsal  in  position,  break  up  into  a 
network  of  sinusoids  about  the  tubules  (see  Chapter  IX). 

The  primary  excretory  duct  or  rnesonephric  duct  is  solid  in  4.25  mm.  em- 
bryos. A  lumen  is  formed  at  7  mm.  wider  opposite  the  openings  of  the  tubules. 
The  duct  is  important  as  from  it  grows  out  the  ureteric  anlage  of  the  permanent 
kidney,  while  the  duct  itself  is  transformed  into  the  genital  duct  of  the  male.,  and 


TIIK    MF.SOXKPIIROS 


207 


its  derivatives.  The  mesonephros  is  probably  not  a  functional  excretory  organ 
in  human  embryos  for  its  tubules  degenerate  before  the  metanephros  becomes 
functional.  It  may  have  some  other  function  and  produce  an  internal  secretion. 
Degeneration  proceeds  rapidly  in  embryos  between  10  and  20  mm.  long,  begin- 
ning cranially.  New  tubules  are  formed  at  the  same  time  caudally.  In  all,  83 
pairs  of  tubules  arise,  of  which  only  26  pairs  persist  at  21  mm.,  and  these  are  in- 
terrupted at  the  connecting  points  between  the  collecting  and  secretory  regions. 
They  are  divided  into  an  upper  group  and  a  lower  group.  The  upper  group, 
numbering  5  to  12,  unites  with  the  rete  tubules  of  the  testis  or  ovary.  In  the 
male  they  form  the  efferent  ductuli  of  the  epididymis.     In  the  female  they  con- 


Supra  renal  gland. 


Dowman's 
Capsule 


Collecting  tubule 
Secretory- tubule 


Mesonephric, 
duct    r 


Muel/erian 
duct 
-/inlage  genital  gland. 


Fig.  201. — Reconstruction  of  a  mesonephric  tubule,  glomerulus  and  mesonephric  duct  from  a  12  mm. 
human  embryo  as  seen  in  transverse  section.     X  95.     Post.card.  v.,  posterior  cardinal  vein. 

stitute  part  of  the  e poop hor on.  Of  the  lower  group  a  few  tubules  persist  in  the 
male,  as  the  paradidymis  with  its  canaliculus  aberrans.  In  the  female  they  form 
the  paroophoron. 


THE  METANEPHROS 
The  essential  parts  of  the  permanent  kidney  are  the  renal  corpuscles  (glom- 
erulus with  Bowman's  capsule),  secretory  tubules  and  collecting  tubules.  The 
collecting  tubules  open  into  expansions  of  the  duct,  the  pelvis  and  calyces.  The 
duct  itself  is  the  ureter,  which  opens  into  the  bladder.  Like  the  mesonephros, 
the  metanephros  is  of  double  origin.  The  ureter,  pelvis,  calyces  and  collecting 
tubules  are  outgrowths  of  the  mesonephric  duct.-  The  secretory  tubules  and  the 
capsules  of  the  renal  corpuscles' are  differentiated  from  the  caudal  end  of  the 
nephrogenic  cord  and  thus  have  the  same  origin  as  the  mesonephric  tubules. 


2o8 


UROGENITAL    SYSTEM 


Wolffian  duel 


hind-gut 


outer  zone 
inner  zone 


.pelvis  of 
kidney 


In  embryos  of  4.5  to  5.5  mm.  the  mesonephric  duct  makes  a  sharp  bend  just 
before  it  joins  the  cloaca  and  it  is  at  the  angle  of  this  bend  that  the  ureteric  anlage 
of  the  metanephros  appears,  dorsal  and  somewhat  median  in  position  (Fig.  209 
B,  C).  The  bud  grows  at  first  dorsally,  then  cranially.  Its  distal  end  expands 
and  forms  the  primitive  pelvis.  Its  proximal  elongated  portion  is  the  ureter. 
The  anlage  grows  into  the  lower  end  of  the  nephrogenic  cord  which,  in  4.6  mm.  em- 
bryos, is  separated  from  the  cranial  end  of  the  cord  at  the  twenty-seventh  seg- 
ment. The  nephrogenic  tissue  forms  a  cap  about  the  primitive  pelvis  and,  as  the 
pelvis  grows  cranially,  is  carried  along  with  it  (Fig.  202).  In  embryos  of  9  to  13 
mm.  the  pelvis  has  reached  a  position  in  the  retroperitoneal  tissue  dorsal  to  the 

mesonephros  and  opposite 
the  second  lumbar  segment. 
Thereafter,  the  kidney  grows 
both  cranially  and  caudally 
without  shifting  its  position. 
The  ureter  lengthens  as  the 
embryo  grows  in  length.  The 
cranial  growth  of  the  kidney 
takes  place  dorsal  to  the 
suprarenal  gland  (Fig.  225). 

Primary  renal  tubules 
grow  out  from  the  primitive 
pelvis  in  10  mm.  embryos. 
Of  the  first  two,  one  is  cranial, 
the  other  caudal  in  position, 
and  between  these  are  two  to 
four  others  (Fig.  203  B,  C). 
From  an  enlargement,  the  ampulla,  at  the  end  of  each  primary  tubule  grow  out 
two,  three  or  four  secondary  tubules.  These  in  turn  give  rise  to  tertiary  tubules 
(Fig.  203  D)  and  the  process  is  repeated  until  the  fifth  month  of  fetal  life,  when 
it  is  estimated  that  twelve  generations  of  tubules  have  been  developed.  The 
pelvis  and  both  primary  and  secondary  tubules  enlarge  during  development. 
The  first  two  primary  tubules  become  the  major  calyces,  and  the  secondary 
tubules  opening  into  them  form  the  minor  calyces  (Fig.  204).  The  tubules  of 
the  third  and  fourth  orders  are  taken  up  into  the  walls  of  the  enlarged  second- 
ary tubules  so  that  the  tubules  of  the  fifth  order,  20  to  30  in  number,  open 
into  the  calyces  minor  as  papillary  ducts.     The  remaining  orders  of  tubules 


cloaeal  membrane 


Fig.  202. — Reconstruction  of  the  anlages  of  the  meta- 
nephros ( after  Schreiner).  The  layers  lettered  inner  and 
outer  zones  constitute  the  nephrogenic  tissue  of  the 
metanephros. 


THE    METANEPHROS 


209 


*5e  Conakry 

Col/ecTiny 

tubules 


constitute  the  collecting  tubules  which  form  the  greater  part  of  the  medulla  of 
the  adult  kidney. 

When  the  four  to  six  primary  tubules  develop,  the  nephrogenic  cap  about  the 
primitive  pelvis  is  subdivided  and  its  four  to  six  parts  cover  the  end  of  each  pri- 

D 

Cranial  pole 
Zubute 


VzntraL  central 

tubule 


Cranial 
pole  tubule 


Caudal 
pole  rubule 

Ureter 


Tertiary  col- 
lecting tubule 

Fig.  203. — Diagrams  showing  the  development  of  the  primitive  pelvis,  calyces  and  collecting  tubules  of 
the  metanephros  (based  on  reconstructions  by  Schreiner  and  Felix). 

mary  tubule.  As  new  orders  of  tubules  arise,  each  mass  of 
nephrogenic  tissue  increases  in  amount  and  is  again  sub- 
divided until  finally  it  forms  a  peripheral  layer  about  the 
ends  of  the  branches  tributary  to  a  primary  tubule.  The 
converging  branches  of  such  a  tubular  "tree"  constitute  a 
primary  renal  unit,  or  pyramid,  with  its  base  at  the  periphery 
of  the  kidney  and  its  apex  projecting  into  the  pelvis.  The 
apices  of  the  pyramids  are  termed  renal  paj)ill(e_  and  through 
them  the  larger  collecting  ducts  open.  The  nephrogenic 
tissue  forms  the  cortex  of  the  kidney,  and  each  sub-division 
of  it,  covering  the  tubules  of  a  pyramid  peripherally,  is  marked 
off  on  the  surface  of  the  organ  by  grooves  or  depressions.  The 
fetal  kidney  is  thus  distinctly  lobated,  the  lobations  persist- 
ing until  after  birth.  The  primary  pyramids  are  subdivided 
into  several  secondary  and  tertiary  pyramids.  Between  the 
pyramids  the  cortex  of  nephrogenic  tissue  dips  down  to  the 
pelvis,  forming  the  renal  r.nl.umtm  fpf  "RprtiniV  The  collect- 
14 


Fig.  204. — The 
pelvis,  calyces,  and 
their  branches  and  a 
portion  of  the  ure- 
ter, from  the  meta- 
nephros of  a  16  mm. 
embryo  (Huber). 


2IO 


UROGENITAL   SYSTEM 


ing  tubules,  on  the  other  hand,  extend  out  into  the  cortex  as  the  cortical  rays 
or  pars  radiata  of  the  cortex.  In  these  rays  and  in  the  medulla  of  the  kidney 
the  collecting  tubules  run  parallel  and  converge  to  the  papilke, 


DIFFERENTIATION  OF  THE  NEPHROGENIC  TISSUE 

In  stages  from  13  to  19  mm.  the  nephrogenic  tissue  about  the  ends  of  the 
collecting  tubules  condenses  into  spherical  masses  which  lie  in  the  angles  between 
the  buds  of  new  collecting  tubules  and  their  parent  stems  (Fig.  205).     One  such 


Fig.  205. — Semidiagrammatic  figures  of  the  anlage  and  differentiation  of  renal  vesicles  and  early 
developmental  stages  of  uriniferous  tubules  of  mammals,  i  and  2,  anlage  and  successive  stages  in  the 
differentiation  of  renal  vesicles,  as  seen  in  sagittal  sections;  3,  section  and  outer  form  of  tubular  anlage 
before  union  with  collecting  tubule  at  the  beginning  of  S-shaped  stage;  4  and  5,  successive  stages  in  the 
development  of  the  tubules,  Bowman's  capsule  and  glomerulus  beginning  with  a  tubular  anlage  showing 
a  well  developed  S-shape  (Huber). 

S\f  ymetancphric  sphere  is  formed  for  each  new  tubule.  The  spheres  are  converted 
^^  into  vesicles  with  eccentrically  placed  lumina.  The  vesicle  elongates,  its  thicker 
?  outer  wall  forming  an  S-shaped  tubule  which  unites  with  a  collecting  tubule, 


DIFFERENTIATION   OF   THE    NEPHROGENIC    TISSUE 


211 


Proximal  convoluted 
tubule 

Distal  convoluted 

tubule. 
Renal  corpuscle 

Connecting  piec  e 


Ascending  limb 
of  Henle's  loop 


Descend/tig  limb 
of  Henle's  loop 

Large  collecting 
tubule 


Arch  of  collecting  tubule 


Arch  of  Col/ecfinq 
tubule  7 

Dteta)  convoluted 

tubule 
Stoerck  3  loop 

Proximal  conuolut- 
tubule 
Connecting  piece 
Glomerulus 

Bowman's  capsule 


Arch  of  collecting 
tubule  y 

Proximal  convoluted 
tubule 

Distal  Convoluted 
tubule , 
Connecting  piece 

Glomerulus 

Bowman's  capsule 
Stoercfc's  loop 


Fig.  206. — Diagrams  showing  the  differentiation  of  the  various  parts  of  the  uriniferous  tubules  of 
the  metanephros  (based  on  the  reconstructions  of  Huber  and  Stoerck).  A.  from  an  adult  human  kid- 
ney;  B,  C,  from  human  embryos. 

its  thin  inner  wall  becoming  the  capsule  (Bowman's)  of  a  renal  corpuscle.  The 
uriniferous  tubules  of  the  adult ' 
kidney  have  a  definite  and 
peculiar  structure  and  arrange- 
ment (Fig.  207  A).  Beginning 
with  a  renal  corpuscle,  each 
tubule  forms  a  proximal  con- 
voluted portion,  a  U-shaped  loop 
(of  Henle)  withi  descending  and 
v  ascending  limbs,*  a  connecting 
piece,  which  lies  close  to  the 
renal  corpuscle,  and  a  distal 
convoluted  portion  continuous 
with  the  collecting  tubule. 
These  parts  are  derived  from 
the  S-shaped  anlage,  which  is 
composed  of  a  lower,  middle  and 


nr> 


Fig.  207. — Diagram  showing  the  relation  of  Bow- 
man's capsule  and  the  uriniferous  tubules  to  the  collecting 
tubules  of  the  metanephros  (Huber).  c ,  collecting  tubules; 
c,  end  branches  of  collecting  tubules;  m,  renal  corpuscles; 
11,  neck;  pc,  proximal  convoluted  tubule;  pi,  descending 
limb  of  Henle's  loop;  al,  ascending  limb  of  Henle's  loop; 
dc,  distal  convoluted  tubule;  /,  junctional  tubule. 


212 


UROGENITAL   SYSTEM 


upper  limb.  The  middle  limb,  somewhat  U-shaped,  bulges  into  the  concavity  of 
Bowman's  capsule  (Fig.  206  B).  By  differentiation  the  lower  portion  of  the 
lower  limb  becomes  Bowman's  capsule,  ingrowing  arteries  forming  the  glomerulus. 
The  upper  part  of  the  same  limb  by  enlargement,  elongation  and  coiling  becomes 


Fig.  208.— Several  stages  in  the  development  of  the  uriniferous  tubules  and  glomeruli  of  the  human 
metanephros  (reconstructions  by  Huber). 

the  proximal  convoluted  tubule.  The  neighboring  portion  of  the  middle  limb 
forms  the  primitive  loop  (of  Stoerck) ;  the  base  of  the  middle  limb  gives  rise  to  the 
connecting  piece,  and  the  rest  of  it,  with  the  upper  limb  of  the  S,  forms  the  distal 
convoluted  tubule  (intermediate  piece  of  Felix).     The  primitivejoop  of  Stoerck 


CLOACA,    BLADDER,    URETHRA   AND   UROGENITAL   SIMS  213 

includes  both  the  ascending  and  descending  limbs  of  Henle's  loop  and  a  portion 
of  the  proximal  convoluted  tubule.  Henle's  loop  is  differentiated  during  the 
fourth  fetal  month  (Toldt)  and  extends  from  the  pars  radiata  of  the  cortex  into 
the  medulla  (Fig.  207).  The  concavity  of  Bowman's  capsule,  into  which  grow 
the  arterial  loops  of  the  glomerulus,  is  at  first  shallow.  Eventually  the  walls  of 
the  capsule  grow  about  and  enclose  the  vascular  knot,  except  at  the  point  where 
the  arteries  enter  and  emerge  (Fig.  205,  4  and  5).  Renal  corpuscles  are  first 
fully  formed  in  28  to  30  mm.  embryos.  The  new  corpuscles  are  formed  peripher- 
ally  from  persisting  nephrogenic  tissue,  hence  in  the  adult  the  oldest  corpuscles 
are  those  next  the  medulla.  Reconstructions  of  the  various  stages  in  the  de- 
velopment of  the  uriniferous  tubules  are  shown  in  Fig.  208. 

Renal  Arteries. — One  or  more  of  the  mesonephnc  arteries  is  transformed  into  the 
renal  artery  of  the  metanephros.  As  any  one  of  the  mesonephric  arteries  may  thus  form  the 
renal  artery,  and  as  they  anastomose,  the  variation  of  the  renal  vessels  both  as  to  position  and 
number  is  accounted  for. 

Anomalies. — If  the  uriniferous  tubules  fail  to  unite  with  the  collecting  tubules,  cystic 
degeneration  may  take  place.  The  cystic  kidneys  of  pathology  may  thus  be  produced.  The 
nephrogenic  tissue  of  the  paired  kidney  anlages  may  fuse,  resulting  in  the  union  of  their  cortex. 
Double  or  triple  ureters  are  sometimes  present. 

DIFFERENTIATION  OF  CLOACA,  BLADDER,  URETHRA  AND  UROGENITAL  SINUS 
In  embryos  of  3  to  4  mm.  the  cloaca,  a  caudal  expansion  of  the  hind-gut,  is 
in  contact  ventrally  with  the  ectoderm,  and  ectoderm  and  entoderm  together 
form  the  cloacal  membrane  (Fig.  209  A).  Ventro-cranially  the  cloaca  gives  off 
the  allantoic  stalk.  At  a  somewhat  later  stage,  the  cloaca  receives  laterally  the 
mesonephric  ducts  and  caudally  is  prolonged  as  the  tail-gut  (Fig.  209  B). 

In  embryos  of  5  mm.  the  ureteric  anlages  of  the  metanephroi  are  present  as 
buds  of  the  mesonephric  ducts  (Fig.  209  C,  D).  Next,  the  saddle-like  partition 
wall  between  the  intestine  and  allantois  grows  caudally.  dividing  the  cloaca 
into  a  dorsal  rectum  and  ventral  primitive  urogenital  sinus.  The  division  is 
complete  in  embryos  of  11  to  15  mm.  and  at  the  same  time  the  partition,  fusing 
with  the  cloacal  membrane,  divides  it  into  the  anal  membrane  of  the  gut  and  the 
urogenital  membrane.  At  11  mm.,  according  to  Felix,  the  primitive  urogenital 
sinus  by  elongation  and  constriction  is  differentiated  into  two  regions:  (1)  a 
dorsalyesico-urethral  anlage  which  receives  the  allantois  and  mesonephric  duct, 
and  is  connected  by  the  constricted  portion  with  (2)  the  phallic  portion  of  the 
urogenital  sinus  (Figs.  210  and  211).  The  latter  extends  into- the  phallus  of 
both  sexes  and  forms  a  greater  part  of  the  urethra  (Fig.  212).     The  vesica- 


214 


UROGENITAL   SYSTEM 


Hind-cut 

Mesonephric 
duct 


Allantois 
Cloaca 


Ctoacal  membrane 


Cloacal  membrane 


Mesonephric 
duct 


Mesonephric  duct 
Allantois 


Hind-cut 


Cloaca 


Metanephros 

'loacat  membrane  yS 

Cloaca 


Tail-out 


Cloaca!  , 
membrane 
Tail-qut 


Fig.  209. — Four  stages  showing  the  differentiation  of  the  cloaca  into  the  rectum,  urethra  and  blad- 
der. A,  from  an  embryo  of  3.5  mm.;  B,  a  somewhat  later  stage;  C,  from  a  5  mm.  embryo;  D,  from  an 
embryo  of  7  mm.  (after  reconstructions  by  Pohlman).     About  100  diameters. 


Mesonephric  duct 
Metanephros 


Inlesline 


Anlage  of  bladder 


Cloacal 
membrane 


Urogenital  sinus 
flee  turn 


Fig.  210. — Reconstructions  from  a  12  mm.  embryo  showing  the  partial  subdivision  of  the  cloaca  into 
rectum  and  urogenital  sinus  (after  Pohlman). 


CLOACA,    BLADDER,    URETHRA   AND    UROGENITAL   SINUS  215 

urethral  anlage  enlarges  and  forms  the  bladder  and  a  portion  of  the  urethra.  In 
7  mm.  embryos  the  proximal  ends  of  the  mesonephric  ducts  are  funnel-shaped, 
and  at  10  mm.,  with  the  enlargement  of  the  bladder,  these  ends  are  taken  up  into 


/llJantois 


Rectum 

Mesonephric  duels. 


Vesica- urethral 
anlage 

Phallic  portion 
of  urogenital  sinus 


Metanephros 


Ureter 


Fig.  211. — Reconstruction  of  the  caudal  portion  of  an  11. 5  mm.  embryo  showing  the  differentiation  of 
the  rectum,  bladder  and  urethra  (Keibel). 


IVIesonephric  To, 
/Inlaye  of  bladder 


(Iterovag, 


Mesonephros 
Mesonephric  duct 

Ureter 
Muellenan  duct 

Mesonephric  duct" 

Rectum 


Spinal  cord 


FlG.  212. — Reconstruction  of  the  caudal  end  of  a  25  mm.  embryo  showing  the  complete  separation  of  the 
rectum  and  urogenital  sinus  and  the  relations  of  the  urogenital  ducts  (after  Keibel's  model). 

its  wall  until  the  ureters  and  mesonephric  ducts  acquire  separate  openings.  The 
ureters,  having  previously  shifted  their  openings  into  the  mesonephric  ducts  from 
a  dorsal  to  lateral  position,  now  open  into  the  vesico-urethral  anlage  lateral  to 
the  mesonephric  ducts.     The  lateral  walls  of  the  bladder  anlage  grow  more  rap- 


tyaWo^ 


2l6 


UROGENITAL   SYSTEM 


idly  than  its  dorso-median  urethral  wall,  hence  the  ureters  are  carried  cranially 
and  laterally  upon  the  wall  of  the  bladder,  while  the  mesonephric  ducts,  now  the 
male  ducts,  open  close  together  into  the  dorsal  wall  of  the  urethra  (Fig.  212). 
The  fate  of  the  phallic  portion  of  the  urogenital  sinus  is  described  on  p.  234  in 
connection  with  the  external  genitalia. 

The  allantois  between  the  bladder  and  the  umbilicus  is  known  as  the  urachus.  Usually 
the  epithelium  of  the  urachus  degenerates,  but  portions  may  persist  and  produce  cysts.  In 
some  cases  it  forms  after  birth  a  patent  tube  opening  at  the  umbilicus.  Its  connective  tissue 
layers  always  persist  as  the  fibrous  lig.  vesico-umbilicale  medium. 

The  transitional  epithelium  of  the  bladder  appears  at  60  mm.  The  outer  longitudinal 
layer  of  small  muscle  develops  in  22  mm.  embryos,  and  in  26  mm.  embryos  the  circular  muscle 
appears.  The  inner  longitudinal  muscle  layer  is  found  at  55  mm.  and  the  sphincter  vesicae 
in  embryos  of  90  mm. 


Mesencephalon 


Prosencephalon 


Rhombencephalon 


R.  lung 


Mesonephric  fold 


Lower  extremity 


Gen  Hal  eminence 


Fig.  21.3. — Ventral  view  of  the  urogenital  folds  in  a  human  embryo  of  five  weeks  (Kollmann's  Atlas). 


THE  GENITAL  GLANDS  AND  DUCTS— INDIFFERENT  STAGE 
As  to  origin  and  early  development,  the  ovary  and  testis  are  alike.     The 
urogenital  fold  (see  p.  205)  is  the  anlage  of  both  the  mesonephros  and  the  genital 
gland  (Fig.  213).     At  first  two-layered,  its  epithelium  in  embryos  of  5  mm.  thick- 


THE    GENITAL   GLANDS    AND    DUCTS— INDIFFERENT   STAGE 


217 


ens  over  the  ventro-median  surface  of  the  fold,  becomes  many-layered  and  bulges 
into  the  ccelom  ventrally,  producing  the  longitudinal  genital  fold.  The  genital 
fold  thus  lies  mesial  and  parallel  to  the  mesonephric  fold.  At  10  to  12  mm.  the 
genital  epithelium  shows  no  sexual  differentiation  (Fig.  214).  There  is  a  super- 
ficial epithelial  layer  and  an  inner  epithelial  mass  of  somewhat  open  structure. 

Owing  to  the  great  development  of  the  suprarenal  glands  and  metanephroi, 
the  cranial  portions  of  the  urogenital  folds,  at  first  parallel  and  close  together,  are 
separated.  This  produces  a  double  bend  in  the  fold  and  in  20  to  25  mm.  embryos 
the  fold  shows  a  cranial  longitudinal  portion,  a  transverse  middle  portion  between 
the  bends,  and  a  longitudinal  caudal  portion.     In  the  last  named,  the  mesonephric 


Lat.  body  wall 

Post,  cardinal 

Suprarenal 
gland 
Glomerulus 


Mesonephric 
duel- 


Inner  epithet,  mass 
ofqemtaL  gland. 

Epithelium  of" 

genital  gland 


Mesentery 


Fig.  214. — Transverse  section  through  the  mesonephros,  genital  gland  and  suprarenal  gland  of  the  right 
side;    from  a  12  mm.  human  embryo.     X  165. 


ducts  course  to  the  cloaca  and  here  the  right  and  left  folds  fuse,  producing  the 
genital  cord  (Fig.  225).  As  the  genital  glands  increase  in  size  they  become  con- 
stricted from  the  mesonephric  fold  by  lateral  and  ventral  grooves  until  the  origi- 
nally broad  base  of  the  genital  fold  is  converted  into  a  stalk  (Fig.  220).  This 
stalk-like  attachment  extends  lengthwise  and  forms  in  the  male  the  mesorchium. 
in  the  female  the  mesovarium.  The  urogenital  fold  is,  at  the  same  time,  constricted 
from  the  dorsal  body  wall  until  it  is  attached  only  by  a  narrow  mesentery  which 
eventually  forms  either  the  lig.  testis  or  lig.  ovarii. 

The  Indifferent  Stage  of  the  Genital  Ducts.— The  mesonephric  ducts,  with 
the  degeneration  of  the  mesonephroi,  become  the  male  genital  ducts.     In  both 


2l8 


UROGENITAL    SYSTEM 


sexes  there  also  develop  a  pair  of  female  ducts  (of  Mueller) .  In  embryos  of  10  to 
ii  mm.  the  Muellerian  ducts  develop  as  ventro-lateral  thickenings  of  the  uro- 
genital epithelium  at  the  level  of  the  third  thoracic  segment  and  near  the  cranial 
ends  of  the  mesonephroi.  Next,  a  ventro-lateral  groove  appears  in  the  epi- 
thelium of  the  mesonephric  fold  (Fig.  215  A) .  Caudally,  the  dorsal  and  ventral 
lips  of  the  groove  close  and  form  a  tube  which  separates  from  and  lies  beneath  the 
epithelium  (Fig.  215  B).  Cranially,  the  tube  remains  open  as  the  funnel-shaped 
ostium  abdominale  of  the  Muellerian  duct.  The  solid  end  of  the  tube  grows 
caudalward  beneath  the  epithelium,  lateral  to  the  mesonephric  or  male  ducts 


Lateral  body  wall — -g. 
Muellerian  groove 


W- Mesentery 


Mesonephric 
tubule 


Genital  aland 


-Anlac/e  of 
Muellerian  duct 


Fig.  215. — Transverse  sections  through  the  anlage  of  the  right  Muellerian  duct  from  a  10  mm. 
embryo.  A,  showing  the  groove  in  the  urogenital  epithelium;  B,  three  sections  caudad  showing  the 
tubular  anlage  of  the  duct.     X  250. 


(Figs.  216  and  218).  Eventually,  by  way  of  the  genital  cord,  the  Muellerian 
ducts  reach  the  median  dorsal  wall  of  the  urogenital  sinus  and  open  into  it. 
Their  further  development  into  uterine  tubes,  uterus  and  vagina  is  described  on 
page  226.  Embryos  not  longer  than  12  mm.  are  thus  characterized  by  the  pos- 
session of  indifferent  genital  glands,  and  of  both  male  and  female  genital  ducts. 
There  is  as  yet  no  sexual  differentiation.  The  development  and  position  of  the 
Muellerian  ducts  is  well  shown  in  ventral  dissections  of  pig  embryos  (Figs.  216 
and  217).     Note  the  enormous  size  of  the  mesonephroi. 

Differentiation  of  the  Testis. — In  male  embryos  of  13  to  15  mm.  the  genital 


TILE    GENITAL    GLANDS    AND    DUCTS— TESTIS 


219 


6enital 
aland. 

Colon 
Allantois 


Umbi/icaL 
artery 


Fig.  2 16.— Ventral  dissection  of  an  iS  mm.  pig  embryo  to  show  the  anlages  of  the  Muellerian  ducts.    X  7. 


Tracht 


/ 


Luna 


Esophajui 


Genital 
qLand     ' 

Mesonephnc 
duct 


Allanto, 
(Bladder) 


Mesonephm 


Muellerian 


Umbilical 
artery 


FlG.  217. — Ventral  dissection  of  a  24  mm.  embryo  showing  the  anlages  of  the  Muellerian  ducts  at  a 
later  stage  of  development  than  in  Fig.  216.     X  6. 


2  20 


UROGENITAL   SYSTEM 


glands  show  two  characters  which  mark  them  as  testes:  (i)  The  occurrence  of 
branched  anastomosing  cords  of  cells,  the  testis  cords;  (2)  the  occurrence  between 
epithelium  and  testis  cords  of  a  layer  of  tissue,  the  anlage  of  the  tunica  albuginea 
(Fig.  218).  According  to  Felix,  the  testis  cords  are  developed  suddenly  from  the 
loose  inner  epithelial  mass  by  a  condensation  of  its  cells.  The  cords  converge  and 
grow  smaller  towards  the  mesorchium,  where  there  is  formed  the  undivided  epi- 
thelial anlage  of  the  rete  testis.  Two  or  three  layers  of  loosely  arranged  cells 
between  the  testis  cords  and  the  epithelium  constitute  the  anlage  of  the  tunica 
albuginea.  According  to  Allen  (Amer.  Jour.  Anat.,  vol.  3,  1904),  the  testis  cords 
are  formed  as  active  ingrowths  of  cords  of  cells  from  the  epithelium. 


Mesentery 


Mesonephric 
tubule 

Mesorchium 


Intermediate  cord 


is  cord 


thelium 

albuginea. 

Fig.  218.— Transverse  section  through  the  left  testis  and  mesonephros  of  a  20  mm.  embryo.     X  250. 


The  testis  cords  soon  become  rounded  and  are  marked  off  by  connective 
tissue  sheaths  from  the  intermediate  cords,  columns  of  undifferentiated  tissue 
which  lie  between  them  (Fig.  219).  Toward  the  rete  testis  the  sheaths  of  the 
testis  cords  unite  to  form  the  anlage  of  the  mediastinum  testis.  The  testis  cords 
are  composed  chiefly  of  indifferent  cells  with  a  few  larger  genital  cells.  The  cells 
gradually  arrange  themselves  radially  about  the  inside  of  the  connective  tissue 
sheath  as  a  many-layered  epithelium  in  which  during  the  seventh  month  a  lumen 
appears.  The  lumina  appear  in  the  peripheral  ends  of  the  testis  cords  and  ex- 
tending toward  the  rete  testis  meet  lumina  which  have  formed  there.  Thus  the 
solid  cords  of  both  are  converted  into  tubules.     The  distal  portions  of  the  testis 


THE    GENITAL   GLANDS   AND    DUCTS — OVARY  221 

tubules  anastomose  and  form  the  tubuli  contorti.  Their  proximal  portions  remain 
straight  as  the  tubuli  recti.  The  rete  testis  becomes  a  network  of  small  tubules 
which  finally  unite  with  the  collecting  tubules  of  the  mesonephros  (see  p.  225). 

The  primitive  genital  cells  of  the  testis  cords  form  the  spermatogonia  of  the 
spermatic  tubules  and  from  these  at  puberty  are  developed  the  spermatogonia 
(p.  24).  The  indifferent  cells  of  the  tubules  become  the  sustentacular  cells  (of 
Sertoli)  of  the  adult  testis.  Primitive  genital  cells  of  the  intermediate  cords  are 
transformed  into  large  pale  cells  which,  after  puberty,  are  numerous  in  the  inter- 
stitial connective  tissue  and  hence  are  called  interstitial  cells.  The  intermediate 
cords  themselves  are  resorbed,  but  the  connective  tissue  sheaths  of  the  tubules 
unite  to  form  septula  which  extend  from  the  mediastinum  testis  to  the  tunica 


Mesorchium 


Ductus  deferens 

Epithelium 


Rektest, 


Fig.  219. — Section  through  the  testis  of  a  100  mm.  fetus.     X  44. 

albuginea.     The  latter  becomes  a  relatively  thick  layer  in  the  adult  testis  and  is  so 
called  because  of  its  whitish  appearance. 

Anomalies. — The  testis  may  be  congenitally  absent,  the  glands  may  be  fused  or  they  may 
fail  to  descend  into  the  scrotum  (cryptorchism).  Duplications  of  the  testis  are  of  rare  occur- 
rence. 

The  Differentiation  of  the  Ovary. — The  primitive  ovary,  like  the  testis, 
consists  of  an  inner  epithelial  mass  and  an  outer  epithelial  layer.  Much  more 
slowly  than  in  the  testis  the  ovarian  characters  appear.  In  embryos  of  50  to  80 
mm.  the  inner  epithelial  mass  composed  of  indifferent  and  primitive  genital  cells 
becomes  less  dense  centrally  and  bulges  into  the  mesovarian  (Fig.  220).  There 
may  be  distinguished  a  dense  outer  cortex  beneath  the  epithelium,  a  clearer 
medullary  zone  containing  large  genital  cells,  and  a  dense  cellular  anlage  in  the 
mesovarium,  the  primitive  rete  ovarii,  which  is  the  homologue  of  the  rete  testis. 


222 


UROGENITAL   SYSTEM 


No  epithelial  cords  and  no  tunica  albuginea  are  developed  at  this  stage,  as  in  the  testis. 
Later,  three  important  changes  take  place:  (i)  There  is  an  ingrowth  of  connec- 
tive tissue  and  blood-vessels  from  the  hilus,  resulting  in  the  formation  of  a  media- 
stinum and  of  septula.     (2)  Most  of  the  cells  derived  from  the  inner  epithelial 


Tubules  of 
mesone.phros 
(Paroophoron) 


Uterine  tube 


Epi'i f he  Hum 
Cortex  - 


Genital  cells 


—Medulla. 


Fig.  220. — Section  of  an  ovary  from  a  65  mm.  embryo.     X  44 

Primordial  egg 


Primordial  ovum 


Blood-vessel 


Germinal  epithelium 
Tunica  albuginea 

Pfliiger's  egg-tubes 


Primordial  ova 


\jft»  fc  *V'.?5  V  "■*'  #*      **»*  ja.  £*    Q4.flvuPf:n    m  w  J 
FlG.  221.— Ovary  of  five-months'  fetus,  showing  egg-tubes  and  primordial  follicles  (De  Lee). 


THE    GENITAL   GLANDS   AND   DUCTS — OVARY 


223 


mass  are  transformed  into  young  ova,  the  process  extending  from  the  rete  ovarii 
peripherally  (Fig.  221).  (3)  In  embryos  of  from  80  to  180  mm.  length  the  ovary 
grows  rapidly,  owing  to  the  formation  of  a  new  peripheral  zone  of  cells,  derived 
in  part  from  the  epithelium.  At  the  end  of  this  period  the  epithelial  cells  beneath 
the  epithelium  are  gradually  replaced  by  a  fibrous  stroma,  the  anlage  of  the  tunica 
albuginea.  Hereafter,  although  folds  of  the  epithelium  are  formed,  these  do  not 
penetrate  beyond  the  tunica  albuginea,  and  all  cells  derived  from  this  source 
subsequently  degenerate.  In  late  fetal  life,  according  to  Felix,  the  so-called 
"germinal  epithelium"  does  not  give  rise  to  primitive  ova. 


§■ 


fa 


mwr T  *-" 

'  ,'ijB     -lifAl-i'-. Membrana 

■«*3f/    J/i[  -    '  granulosa 


imordial       Vitell 


follicle 


men 
brane 


I* 


0 


\% 


v 


v^ 


Fig.  222. — Primordial  ova  and  early  stages  in  the  development  of  the  Graafian  follicle  (De  Lee's 

Obstetrics). 


Coincident  with  the  origin  of  a  new  zone  of  cells  at  the  periphery  of  the  ovary 
goes  the  degeneration  of  young  ova  in  the  medulla.  By  the  ingrowth  into  this 
region  of  connective  tissue  septa,  the  ova  are  separated  into  clusters  or  cords,  the 
genital  cells  of  which  all  degenerate,  leaving  in  the  medulla  only  a  stroma  of  con- 
nective tissue.  Late  in  fetal  life  the  indifferent  cells,  by  surrounding  young  ova, 
produce  primordial  follicles  (Fig.  222  A).  During  the  first  year  after  birth  the 
primitive  follicles  are  transformed  into  the  vesicular  Graafian  follicles.  By  cell 
division  the  follicle  cells  form  a  zone  many  layers  deep  about  the  young  ovum 
(Fig.  222  B).  Next  a  cavity  appears  in  the  sphere  of  follicle  cells,  enlarges  and 
produces  a  vesicle  filled  with  fluid  (Fig.  223).     The  ovum  is  now  eccentrically 


224 


UROGENITAL   SYSTEM 


located  and  the  follicle  cells  directly  surrounding  it  constitute  the  cumulus 
oophorus  (egg-bearing  hillock).  About  the  stratum  granulosum  formed  by  the 
original  follicle  cells  there  is  differentiated  from  the  stroma  of  the  ovary  the  theca 
joUicidi.  This  is  composed  of  an  inner  vascular  tunica  interna  and  of  an  outer 
fibrous  tunica  externa. 

Fully  formed  Graafian  follicles  are  found  in  the  ovaries  during  the  second 
year  and  they  may  even  be  present  before  birth.  Ovulation  may  occur  at  this 
time  but  usually  these  precociously  formed  follicles  degenerate  with  their  con- 


■Tunica,  inh 


•SiraJtu-ttf 
granulosum 


Fig.  223. — Graafian  follicle  and  ovum  from  the  ovary  of  a  fifteen-year-old  girl.     X  30. 


tained  ova.  Thus,  although  thousands  of  ova  are  produced  in  the  ovary,  only  a 
comparatively  few  are  set  free  ready  for  fertilization  during  the  sexually  active 
life  of  the  female,  from  puberty  to  the  climacteric  period  or  menopause.  The 
relation  of  ovulation  to  menstruation  has  been  discussed  on  p.  96. 

The  Corpus  Lulcum. — After  ovulation  a  blood  clot,  the  corpus  hemorrhagicum,  forms 
within  the  empty  follicle.  The  follicle  cells  of  the  stratum  granulosum  proliferate,  enlarge  and 
produce  a  yellow  pigment.  The  whole  structure,  composed  of  lutein  cells  and  connective  tissue 
strands,  is  termed  the  corpus  lutcum  or  yellow  body.  The  blood  clot  is  resorbed  and  replaced 
by  fibrous  scar  tissue  white  in  color  and  known  as  the  corpus  albicans.  If  pregnancy  does 
not  intervene  the  corpus  luteum  spurium  reaches  its  greatest  development  within  two  weeks 
and  then  degenerates.     In  cases  of  pregnancy  the  true  corpus  lutcum  continues  its  growth  until 


THE    GENITAL    (.LANDS       I'NION    WITH    MKSONEPHROS  225 

at  the  fifth  or  sixth  month  it  reaches  a  maximal  diameter  of  15  to  30  mm.     At  birth  it  is  Mill 
a  prominent  structure  in  the  ovary  and  it  is  believed  to  produce  an  internal  secretion;   for  if 

the  corpus  luteum  is  removed  the  ovum  fails  to  attach  itself  to  the  wall  of  the  uterus. 

The  Rete  Ovarii. — The  cells  of  the  rete  ovarii  remain  compact,  distinct  and 
continuous  only  with  the  stroma  of  the  medulla,  the  medullary  cords.  The  anlage 
is  differentiated  into  a  network  of  solid  cords  in  60  mm.  embryos  (head-foot 
length)  and  these  connect  with  the  collecting  tubules  of  the  mesonephros.  Some 
time  before  birth  lumina  appear  in  the  cords  transforming  them  into  tubules 
homologous  with  those  of  the  rete  testis. 

Anomalies. — The  ovaries  vary  greatly  in  form  and  position.  Congenital 
absence  of  one  or  both  glands  is  rare.  Cases  of  supernumerary  and  bilobed  ovar- 
ies have  been  observed. 

Comparing  the  testis  and  ovary  in  development,  it  is  clear  that  the  superficial 
epithelium  after  forming  the  inner  epithelial  mass  takes  no  further  part  in  the  dif- 
ferentiation of  the  testis  and  only  a  small  part  in  that  of  the  ovary.  The  testis 
cords,  rete  testis  and  tunica  albuginea  are  formed  early  from  the  inner  epithelial 
mass,  which  determines  their  form.  The  inner  epithelial  mass  of  the  ovary  de- 
velops slowly  and  its  passive  cells  are  separated  and  surrounded  by  actively 
ingrowing  connective  tissue.  The  primordial  follicles  when  developed  are  not  the 
homologues  of  the  testis  cords  and  the  tunica  albuginea  appears  late.  The  rete 
ovarii  is  the  homologue  of  the  rete  testis  but  remains  a  rudimentary  structure. 

The  Union  of  the  Genital  Glands  and  Mesonephric  Tubules. — In  both  male 
and  female  embryos  of  21  mm.  the  mesonephros  has  degenerated  until,  according 
to  Felix,  only  twenty-six  tubules  persist  separated  into  a  cranial  and  a  caudal 
group.  In  the  cranial  group  of  5  to  12  tubules  the  collecting  portions  have  sepa- 
rated from  the  secretory  portions.  The  free  ends  of  these  collecting  tubules 
project  against  that  part  of  the  inner  epithelial  mass  which  gives  rise  to  the  rete 
tubules  of  either  testis  or  ovary  (Fig.  220).  The  cords  of  the  rete  develop  in 
contact  with  the  collecting  tubules  of  the  mesonephros  and  unite  with  them.  In 
the  male  this  union  was  observed  by  Felix  in  embryos  of  60  mm.  head-foot  length. 
The  lumina  of  rete  and  collecting  tubules  become  continuous  and  the  latter  are 
transformed  into  the  ductuli  eferentes  of  the  epididymis.  They  convey  the  sperms 
from  the  testis  tubules  into  the  mesonephric  duct,  which  thus  becomes  the  male 
genital  duet.  During  the  fifth  month  of  pregnancy  the  ductuli  efferentes  coil  at 
their  proximal  ends  and  when  surrounded  by  connective  tissue  they  are  known  as 
coni  vaseulosi.  The  cranial  portion  of  the  male  genital  duct  also  coils  and  forms 
the  canalis  epididymis.  Its  blind  cranial  end  persists  as  the  appendix  epididymis. 
15 


226  UROGENITAL   SYSTEM 

The  caudal  portion  of  the  male  duct  remains  straight  and  as  the  ductus  def- 
erens extends  from  the  epididymis  to  the  urethra.  Near  its  opening  into  the 
latter  it  dilates  to  form  the  ampulla  and  from  its  wall  is  evaginated  the  sacculated 
seminal  vesicle  in  embryos  of  60  mm. 

The  epithelium  of  the  genital  duct  is  at  first  a  single  layer  of  cubical  cells.  At  70  mm. 
the  cells  become  columnar  with  non-motile  cilia  at  their  free  ends.  Quite  late  in  development 
the  surrounding  mesenchyma  gives  rise  to  the  muscular  layers. 

In  the  male,  the  rete  testis,  cranial  group  of  mesonephric  collecting  tubules 
and  mesonephric  duct  thus  form  functional  structures  (Fig.  231  C).  The  lower 
group  of  collecting  tubules  persist  as  the  vestigial  paradidymis.  The  Muellerian 
ducts  of  male  embryos  begin  to  retrograde  at  30  mm.  The  middle  portion  of  each 
degenerates  but  its  cranial  end  persists  as  the  appendix  testis;  its  caudal  end  united 
with  its  fellow  forms  a  pouch  in  the  median  dorsal  wall  of  the  urethra.  This  is 
the  homologue  of  the  vagina  of  the  female  and  is  called  the  vagina  masculina. 

In  the  female,  the  rete  ovarii  is  always  a  rudimentary  structure,  yet  some 
time  before  birth  it  unites  with  the  cranially  persisting  group  of  mesonephric 
tubules  and  forms  a  rudimentary  structure,  the  epoophoron  (Fig.  231  B).  In  its 
cords  lumina  appear,  the  epithelial  cells  become  ciliated  and  smooth  muscle 
tissue  is  developed  corresponding  to  that  of  the  epididymis.  Usually  the  greater 
part  of  the  male  genital  ducts  atrophy  in  the  female,  the  process  beginning  at  30 
mm.  Thus  the  tubules  of  the  epoophoron  are  without  an  outlet.  The  caudal 
portions  of  the  male  genital  ducts  persist  as  Gartner's  canals. 

These  may  extend  as  vestigial  structures  from  the  epoophoron  to  the  lateral  walls  of  the 
vagina,  passing  through  the  broad  ligament  and  the  wall  of  the  uterus.  They  open  into  the 
vagina  close  to  the  free  border  of  the  hymen  (R.  Meyer).  The  canals  are  rarely  present  through- 
out their  entire  length  and  are  absent  in  two-thirds  to  three-quarters  of  the  cases  examined. 
It  is  an  interesting  fact  that  in  male  and  female  embryos  the  ducts  of  the  opposite  sex  begin 
to  degenerate  at  the  same  stage,  30  mm. 

The  Uterine  Tubes,  Uterus  and  Vagina. — The  Muellerian,  or  female  ducts, 
after  taking  their  origin  as  described  on  p.  218,  grow  caudally,  following  the  course 
of  the  mesonephric  ducts  (Fig.  217).  At  first  lateral  in  position,  the  Muellerian 
ducts  cross  the  mesonephric  ducts  and  enter  the  genital  cord  median  to  them. 
In  embryos  of  20  to  30  mm.  their  caudal  ends  are  dorsal  to  the  urogenital  sinus, 
extending  as  far  as  the  Muellerian  tubercle,  a  projection  into  the  median  dorsal 
wall  of  the  vesico-urethral  anlage  (Fig.  212).  This  tubercle  marks  also  the  posi- 
tion of  the  future  hymen.  In  embryos  of  70  mm.  the  Muellerian  ducts  break 
through  the  wall  of  the  urethra  and  open  into  its  cavity.     Before  this  takes  place 


THE  GENITAL  GLANDS  AND  DUCTS — UTERUS 


227 


the  caudal  ends  of  the  Muellerian  ducts,  which  are  pressed  close  together  between 
the  mesonephric  ducts  in  the  genital  cord,  fuse,  and  in  both  male  and  female  em- 
bryos of  20  to  30  mm.  give  rise  to  the  unpaired  anlage  of  the  uterus  and  vagina 
(Fig.  224  A).  The  utero-vaginal  anlage  of  the  male  remains  rudimentary.  The 
uterine  portion  of  the  anlage  degenerates  with  the  paired  portions  of  the  Muel- 
lerian ducts.  The  vaginal  portion  remains  as  the  vagina  masculina,  and  the 
extreme  cranial  end  of  each  Muellerian  duct  persists  as  the  appendix  testis. 

As  pointed  out  by  Felix,  the  term  "uterus  masculinus"  as  applied  to  the  remains  of  the 
utero  vaginal  anlage  is  a  misnomer,  for  the  vaginal  portion  of  the  anlage  persists  and  its  uterine 
portion  degenerates. 

Uterus  and  Vagina. — Since  the  Muellerian  ducts  develop  in  the  urogenital 
folds,  they  make  two  bends  in  their  course  corresponding  to  those  of  the  folds 


Cervix 
uteri 


Uterine  tube 


Fundus  of  Uterus 


Found- 
Ligament 

Horizontal,  portion 
of  uterine  tube 


Mesenchyn 


Fig.  224. — Diagrams  showing  the  development  of  the  uterus  and  vagina  (modified  after  Felix). 


(Fig.  224  A).  Each  consists  of  a  cranial  longitudinal  portion,  a  middle  transverse 
portion  and  a  caudal  longitudinal  portion,  which  is  fused  with  its  fellow  to  form 
the  utero-vaginal  anlage.  At  the  angle  between  the  cranial  and  middle  portions 
is  attached  the  inguinal  fold,  the  future  round  ligament  of  the  uterus  (Figs.  225 
and  226).  The  mesenchyma  condenses  about  the  utero-vaginal  anlage  and  the 
middle  transverse  portion  of  the  Muellerian  ducts,  forming  a  thick,  sharply  de- 
fined layer,  from  which  is  differentiated  the  muscle  and  connective  tissue  of  the 
uterus  and  vagina  (Fig.  224  B).  As  development  proceeds  the  cranial  wall  be- 
tween the  transverse  portions  of  the  Muellerian  ducts  bulges  outward  so  that  its 
original  cranial  concavity  becomes  convex  (Fig.  224  B).  The  middle  transverse 
portions  of  the  ducts  are  thus  taken  up  into  the  wall  of  the  uterus  forming  its 


228  UROGENITAL   SYSTEM 

fundus,  while  the  narrow  cervix  of  the  uterus  and  the  vagina  arise  from  the  utero- 
vaginal anlage.  Through  the  differentiation  of  its  mesenchymatous  wall,  the 
uterus  is  first  brought  into  relation  with  the  round  ligament. 

At  50  mm.  the  mesenchyma  begins  to  differentiate  a  connective  layer  tissue.  At  80  mm. 
the  mucosa  and  muscularis  may  be  distinguished.  The  first  circular  muscle  fibers  appear  in 
1  So  mm.  embryos,  the  other  muscle  layers  develop  later.  The  epithelium  of  the  uterine  tubes 
and  the  tubular  portion  of  the  uterus  (fundus)  remains  simple  with  cylindrical  or  cuboidal 
cells.  The  tubular  fundus  glands  of  the  uterus  may  not  appear  until  near  puberty.  At  38 
mm.  the  epithelium  of  the  cervix  and  vagina  becomes  stratified.  The  vagina  is  at  first  without 
a  lumen.  From  the  third  to  the  sixth  months  of  fetal  life  dorsal  and  ventral  outgrowths  of 
the  epithelium  form  the  fornices  of  the  vagina.  The  vaginal  lumen  appears  in  embryos  of 
150  to  200  mm.,  arising  from  the  degeneration  of  the  central  epithelial  cells.  The  fornices 
hollow  out  and  form  the  boundary  line  between  the  cervix  uteri  and  the  vagina.  The  epithelial 
cells  of  the  former  become  stratified  and  cylindrical,  those  of  the  vagina  are  of  the  stratified 
squamous  type.  The  paired  cranial  portions  of  the  Muellerian  ducts  become  the  uterine  tubes. 
The  epithelial  anlages  of  the  Muellerian  ducts  form  the  epithelial  layers  of  the  uterine  tubes 
uterus  and  vagina. 

The  Hymen. — At  the  point  where  the  utero-vaginal  anlage  breaks  through 
the  wall  of  the  urogenital  sinus  there  is  present  the  tubercle  of  Mueller,  which 
marks  the  lower  limits  of  the  vagina.  The  tubercle  is  compressed  into  a  disk 
lined  internally  by  the  vaginal  epithelium,  externally  by  the  epithelium  of  the 
urogenital  sinus.  These  layers  with  the  mesenchyma  between  them  constitute 
the  hymen,  which  thus  guards  the  opening  into  the  vagina.  A  circular  aperture 
in  the  hymen  is  for  a  time  closed  by  a  knob  of  epithelial  cells,  but  later  when  the 
hymen  becomes  funnel-shaped  the  opening  is  compressed  laterally  to  form  a 
sagittal  slit,  the  ostium  vagina. 

The  Growth  of  the  Uterus. — The  uterus  grows  but  slowly  until  near  puberty,  being  about 
the  same  length  (27  mm.)  at  birth  as  in  a  girl  of  nine  years.  Just  before  and  after  puberty 
growth  is  more  rapid,  a  length  of  72  mm.  being  attained  at  18  years.  This  is  nearly  the  maximal 
length  of  the  virginal  uterus. 

Anomalies. — Owing  to  the  complicated  processes  leading  to  their  formation,  many  cases 
of  abnormal  uterus  and  vagina  occur.  A  complete  classification  of  these  cases  is  given  by  Felix 
(Keibel  and  Mall,  vol.  2,  p.  930).  The  more  common  anomalies  are  (1)  complete  duplication 
of  the  uterus  and  vagina  due  to  the  failure  of  the  Muellerian  ducts  to  fuse;  (2)  uterus  bicornis, 
due  to  the  incomplete  fusion  of  the  ducts.  Combined  with  these  defects  the  lumen  of  the 
uterus  and  vagina  may  fail,  partly  or  completely,  to  develop  and  the  vaginal  canal  may  not  open 
to  the  exterior.  (3)  The  body  of  the  uterus  may  remain  flat  (uterus  planifundis)  or  may  fail 
to  grow  to  normal  size  (uterus  fetalis  and  infantilis).  (4)  Congenital  absence  of  one  or  both 
uterine  tubes,  uterus,  and  vagina  rarely  occurs,  but  may  be  associated  with  hermaphroditism 
of  the  external  genitalia. 

The  Ligaments  of  the  Internal  Genitalia. — Female. — The  loose  mesenchyma 
of  the  genital  cord  gives  rise  laterally  to  the  broad  ligaments  of  the  uterus  in 


THE    GENITAL    GLANDS    AND    DUCTS — LIGAMENTS 


229 


females.  In  the  genital  fold,  extending  from  the  caudal  end  of  the  ovary  to  the 
genital  cord,  connective  tissue  and  smooth  muscle  fibers  developing  form  the 
proper  ligament  of  the  ovary.  The  uterus  develops  in  the  genital  cord  so  the  liga- 
ment of  the  ovary  extends  to  the  posterior  surface  of  the  uterine  wall.  In  the 
male  the  homologue  of  the  proper  ligament  of  the  ovary  is  the  ligament  of  the 
testis. 

In  both  sexes  the  inguinal  fold  extends  from  the  urogenital  fold  to  the  crista 
inguinalis,  located  on  the  inside  of  the  ventral  abdominal  wall,  a  point  which 
marks  the  future  entrance  of  the  inguinal  canal.  The  inguinal  fold  thus  forms  a 
bridge  between  the  urogenital  fold  (in  the  middle  portion  of  which  the  uterus 
develops  in  the  female)  and  the  abdominal  wall  at  the  entrance  of  the  inguinal 


Diaphragmatic  by. 
of mesonephros  ' 

Afetanephros 
Ureter 


Chorda,      . 
qubernaculb 


GLcws  of  final  I  us 


Suprarenal 
gland 

Muellerian  duct, 
in  mesonephric  fold 

Genital  gland. 


f\ectum 
Genital  cord. 

Genital  SWeltinQ 


Fig.  225. — Ventral  dissection  of  a  human  embryo  of  23  mm.  showing  the  urogenital  organs.     The  right 
suprarenal  gland  has  been  removed  to  show  the  metanephros. 


canal  (Fig.  225).  In  the  inguinal  crest  is  differentiated  the  conical  anlage  of  the 
chorda  gubemaculi,  which  later  becomes  a  fibrous  cord.  The  abdominal  muscles 
develop  around  it  and  the  external  oblique  muscle  leaves  a  foramen,  through 
which  it  connects  with  a  second  cord  termed  in  the  male  the  lig.  scroti,  in  the 
female  the  lig.  labiale  (Fig.  226).  The  chorda  gubemaculi  and  the  lig.  labiale 
together  constitute  the  round  ligament  of  the  uterus,  as  they  form  a  continuous 
cord  extending  from  the  urogenital  fold  to  the  base  of  the  genital  tubercle.  With 
the  development  of  the  uterus  in  the  urogenital  fold,  the  round  ligament  becomes 
attached  to  its  ventral  surface. 

Male. — The  ligamcntum  testis,  like  the  lig.  ovarii,  develops  in  the  genital 
fold  and  extends  from  the  caudal  end  of  the  testis  to  the  mesonephric  fold  at  a 


230 


UROGENITAL   SYSTEM 


point  opposite  the  attachment  of  the  inguinal  fold.  The  inguina.  fold,  as  we  have 
seen,  is  continuous  with  the  inguinal  crest  and  the  chorda  gubernaculi.  A  cord 
develops  in  the  mesonephric  fold  and  connects  the  ligamentum  testis  with  the 
chorda  gubernaculi,  for  in  the  male  the  uterus  does  not  intervene  between  these 
two.  The  chorda  gubernaculi  is  continued  to  the  integument  of  the  scrotum  by 
way  of  the  ligamentum  scroti.  Thus  there  is  formed  a  continuous  cord,  the 
gubernacnlnm  testis,  extending  from  the  caudal  end  of  the  testis  through  the  in- 
guinal canal  to  the  scrotal  integument.  The  gubernaculum  is  composed  of  the 
ligamentum  testis,  of  a  mesonephric  cord,  of  the  chorda  gubernaculi,  and  of  the  lig. 


Suprarenal 
gland 


Diaphragmatic 
ligament 

Ureter 
Ovary 

Llq.  ovarii 


Bound  liq,  of 
Uterus 


Phall 


—  fvletanephros 

Pelvis  of 
metanephros 

Uterine  tube 

-  Rectum 

Utero-vaqinaL 
anlaae     J 

3ladder 


—  Genital  swelling 


Glans  clitoris 


Fig.  226. — Ventral  dissection  of  a  female  human  embryo  of  34  mm.     The  urogenital  organs  are  dissected 
out  and  the  left   suprarenal  gland  has  been  removed. 


scroti,  and  is  the  homologue  of  the  ovarian  ligament  plus  the  round  ligament  of 
the  uterus. 

The  Descent  of  the  Testis  and  Ovary. — The  original  position  of  the  testis 
and  ovary  is  changed  during  the  later  stages  of  development.  At  first  they  are 
elongate  structures,  extending  in  the  abdominal  cavity  from  the  diaphragm  cau- 
dally  towards  the  pelvis  (Fig.  213).  As  development  proceeds,  their  caudal  ends 
enlarge  and  their  cranial  portions  atrophy  so  that  there  is  a  progressive  movement 
of  the  glands  caudad.  When  the  process  of  growth  and  degeneration  is  completed 
the  caudal  ends  of  the  testis  lie  at  the  boundary  line  between  the  abdomen  and 


THE    GENITAL    GLANDS — DESCENT    OF    TESTIS 


231 


pelvis  while  the  ovaries  are  located  in  the  pelvis  itself,  a  position  which  they  re- 
tain. Owing  to  the  rotation  of  the  ovary  about  its  middle  point  as  an  axis  it 
takes  up  a  transverse  position.  It  also  rotates  nearly  1800  about  the  Muellerian 
duct  as  an  axis  and  thus  comes  to  lie  caudal  to  the  uterine  tube. 

The  testis  normally  leaves  the  abdominal  cavity,  descending  into  the  scro- 
tum. As  described  above,  there  is  early  developed  between  the  testis  and  the 
integument  of,  the  scrotum  a  fibrous  cord,  the  gubemaculum  testis.  Owing  to 
changes  in  the  position  of  the  ventral  abdominal  wall  and  umbilical  arteries, 
changes  connected  with  the  return  of  the  intestinal  coils  into  the  ccelom,  there 
are  formed  in  each  side  of  the  abdominal  wall  sac-like  pockets,  the  anlages  of  the 
vaginal  sacs.  Close  to  each  saccus  vaginalis  lies  the  caudal  end  of  a  testis,  while 
extending  into  the  scrotum  beneath  the  peritoneum  is  the  gubemaculum  testis. 
The  saccus  vaginalis  later  invaginates  into  the  scrotum  over  the  pubic  bone,  carry- 
ing with  it  also  representatives  of  the 
muscular  layers  of  the  abdominal  wall. 
Whether  due  to  the  active  shortening 
or  to  the  unequal  growth  of  the  guber- 
naculum  testis,  the  descent  of  the  testis 
into  the  vaginal  sac  begins  during  the 
seventh  month  of  fetal  life  and  by  the 
eighth  month  or  at  least  before  birth 
the  testis  is  usually  located  in  the  scro- 
tum (Fig.  227).  It  must  be  remem- 
bered that  the  testis  and  gubemaculum  are  covered  by  the  peritoneum  before 
the  descent  begins,  consequently  the  testis  follows  the  gubemaculum  along  the 
inguinal  canal  dorsal  to  the  peritoneum  and,  when  it  reaches  the  scrotum,  is 
invaginated  into  the  saccus  vaginalis.  The  gubemaculum  is  said  to  degenerate 
during  the  descent  of  the  testis  or  immediately  after.  Abnormally,  the  testis 
may  remain  in  the  abdomen,  a  condition  known  as  cryptorchism  (concealed 
testis)  and  associated  with  sterility  in~man.  In  some  mammals  (bat  and 
elephant)  it  is  the  normal  condition. 

Shortly  after  birth  the  inguinal  canal  connecting  the  saccus  vaginalis  with 
the  abdominal  cavity  becomes  solid  and  its  epithelium  is  resorbed.  The  now 
isolated  vaginal  sac  becomes  the  tunica  vaginalis  of  the  testis.  Its  visceral  layer 
is  closely  applied  to  the  testis  and  its  parietal  layer  forms  the  lining  of  the  scrotal 
sac.  The  ductus  deferens  and  spermatic  vessels  are  of  course  carried  down  into 
the  scrotum  with  the  testis  and  epididymis.     They  are  surrounded  by  connective 


Fig.  227. — Descent  of  the  testis  (Cunning- 
ham), ac,  abdominal  cavity;  pv,  processus 
vaginalis;  /,  testis;  5,  scrotum;  tv,  tunica  vagin- 
alis; .v,  rudiment  of  processus  vaginalis. 


232 


UROGENITAL   SYSTEM 


tissue  and,  with  the  spermatic  vessels,  constitute  the  spermatic  cord.  Owing 
to  the  descent  of  the  testis,  the  ductus  deferens  is  looped  over  the  ureter  in  the 
abdomen  (Fig.  23 1  C) .  In  some  cases  the  inguinal  canals  remain  open  so  that  the 
testis  may  slip  back  into  the  abdominal  cavity.  Such  conditions  may  lead  to 
inguinal  hernia  of  the  intestine.  Open  inguinal  canals  occur  normally  in  the 
rabbit. 

THE  EXTERNAL  GENITALIA 
Indifferent  Stage. — In  both  sexes  there  develops  early  in  the  midline  of  the 
ventral  body  wall,  between  the  tail  and  umbilical  cord,  the  cloacal  tubercle  (Fig. 
228).     Upon  this  appears  a  knob-like  structure,  the  phallus,  and  the  two  together 


Fig.  228. — Four  stages  in  the  development  of  the  external  genitalia  in  embryos  of  24  to  34  mm. 
Indifferent  stage:  i,  phallus;  2,  glans;  3,  primitive  urogenital  opening;  4,  genital  tubercle  or  swelling; 
5,  anus;  6,  coccyx  (Tourneux  in  Heisler's  Embryology). 

constitute  the  genital  eminence.  Cranially  about  the  phallus  the  cloacal  tu- 
bercle forms  a  crescent-shaped  genital  tubercle,  which  later  gives  rise  to  the  right 
and  left  genital  swellings.  The  phallus  grows  rapidly  and  into  it  extends  the 
phallic  portion  of  the  urogenital  sinus.  At  the  end  of  the  phallus  the  epithelium 
of  the  sinus  forms  a  solid  urethral  plate  (Fig.  212).  Along  the  anal  surface  of  the 
phallus  in  the  midline,  the  wall  of  the  urogenital  sinus  breaks  through  to  the  ex- 
terior and  forms  the  slit-like  primitive  urogenital  aperture.  In  embryos  of  21  to 
28  mm.,  at  the  end  of  the  phallus,  the  glans  is  marked  off  from  the  base  by  a  cir- 
cular groove,  the  coronary  sulcus  (Figs.  225  and  228  B). 


THE   EXTERNAL   GENITALIA 


233 


Female. — A  deep  groove  appears  about  the  base  of  the  phallus  separating  it 
from  the  genital  tubercle,  which  becomes  a  circular  swelling  (Fig.  229).  From  the 
swelling  differentiates  (1)  cranially,  the  mons  veneris;  (2)  laterally,  the  right  and 
left  labia  major  a;  (3)  caudally,  the  posterior  commissure  of  the  labia  majora. 
The  glans  of  the  phallus  forms  the  glans  clitoris  of  the  female.  On  the  anal  sur- 
face of  the  phallus  beginning  at  the  coronary  sulcus  the  primitive  urogenital  open- 
ing closes  distally,  forming  the  urethral  groove.     Proximally  it  remains  open,  as 


FlG.  229. — Four  stages  in  the  development  of  the  female  external  genitalia  (Tourncux  in  Heisler). 
1,  clitoris;  2,  glans  clitoris;  3,  urogenital  aperture  on  each  side  of  which  are  the  labia  minora  (7);  4, 
labia  majora;   5,  anus;   5,  coccygeal  eminence;    7,  labia  minora. 

the  definitive  urogenital  opening  near  the  base  of  the  phallus.  The  lips  of  this 
groove  and  opening  enlarge  and  become  the  labia  minora.  The  cranial  surface 
of  the  phallus  forms  a  fold,  the  prcpucium,  which,  however,  is  not  the  homologue 
of  the  male  fore-skin.  This  in  the  female  is  represented  by  a  ring-like  rudiment 
at  the  base  of  the  glans  clitoris. 

Male. — The  phallus  grows  rapidly  at  its  base  so  that  the  glans  and  primitive 
urogenital  opening  are  carried  some  distance  from  the  anus  (Fig.  230).     A  cylin- 


234 


UROGENITAL    SYSTEM 


Br* 


B 


drical  fold  of  the  epithelium,  incomplete  on  the  anal  side,  grows  down  into  the 
end  of  the  glans,  which  becomes  the  glans  penis.  By  the  disappearance  of  the 
central  cells  of  the  epithelial  downgrowth  an  outer  cylindrical  mantle,  the  pre- 
puciam  or  fore-skin,  is  formed  about  the  spherical  glans.  Where  the  epithelial 
downgrowth  is  incomplete  the  glans  and  foreskin  remain  connected,  the  persist- 
ing connection  being  the  frenulum 
prepucii. 

The  urogenital  sinus,  as  we  have 
seen,  extends  out  into  the  phallus 
and  in  the  glans  becomes  the  solid 
urethral  plate.  With  the  great 
elongation  of  the  male  phallus,  the 
open  portion  of  the  urogenital  sinus 
also  is  lengthened  and  forms  the 
greater  part  of  the  penile  urethra. 
In  embryos  of  70  mm.  the  groove- 
like  primitive  urogenital  opening, 
located  in  the  male  near  the  glans 
and  distant  from  the  anus,  closes 
and  thus  is  formed  a  further  por- 
tion of  the  urethra.  The  failure  of 
this  opening  to  close  gives  rise  to 
an  anomaly  known  as  hypospadias. 
The  lips  of  the  urogenital  opening, 
it  will  be  remembered,  correspond 
to  the  labia  minora  or  nymphce  of 
the  female.  Finally  at  100  mm. 
the  solid  urethral  plate  of  the 
glans  splits,  forms  a  groove  to  the 
tip  of  the  glans  and  this  groov«4n  turn  is  closed,  continuing  the  urethra  to  the 
definitive  opening  at  the  tip  of  the  glans.  Owing  to  the  rapid  growth  in 
length  of  the  penis,  there  is  formed  between  its  base  and  the  anus  an 
unpaired  area,  termed  by  Felix  the  scrotal  area  as'* it  is  the  anlage  of  the 
scrotum.  At  60  mm.  this  forms  a  median  scrotal  swelling  continuous  with 
the  paired  genital  swellings.  When  the  scrotal  sac  develops  in  the  scrotal  area, 
the  dense  tissue  in  the  median  line  is  compressed  and  forms  the  septum  scroti. 
The  attachment  of  this  septum  forms  an  external  median  depression.     The  testes 


I 


.  v 


c 

Fig.  230.— Four  stages  in  the  development  of  the 
male  external  genitals  (Tourneux  in  Heisler).  1, 
penis;  2,  glans;  3,  urogenital  groove;  4,  urogenital 
swellings  corresponding  to  labia  majora  of  female; 
5,  anus;  6,  coccygeal  eminence;  7,  scrotal  area  with 
perineo-scrotal  raphe. 


THE    EXTERNAL   GENITALIA  235 

descend  into  the  vaginal  sacs  of  the  scrotum  through  the  paired  genital  swellings, 
as  described  on  p.  231,  but  the  scrotum  itself  is  an  unpaired  structure  derived 
from  the  scrotal  area.  After  the  descent  of  the  testes  the  genital  swellings  dis- 
appear. 

Comparing  the  male  and  female  external  genitalia,  it  is  plain  that  the  glans 
penis  and  glans  clitoris  are  homologous.  The  labia  minora  correspond  to  the 
phallic  folds  which  close  about  the  primitive  urogenital  opening  and  the  anal 
surface  of  the  penis.  The  greater  part  of  the  stem  of  the  male  phallus  does  not 
develop  in  the  female.  On  the  other  hand,  the  genital  swellings  enlarge  and  be- 
come the  labia  majora  of  the  female,  while  in  the  male  they  are  only  temporary 
structures.  The  scrotum  does  not  develop  in  the  female,  being  represented  only 
by  the  posterior  commissure  of  the  labia  majora. 

The  Prostate  Gland. — This  is  developed  in  both  sexes  as  several  outgrowths 
above  and  below  the  entrance  of  the  male  ducts  into  the  urogenital  sinus.  The  tu- 
bules arise  in  five  distinct  groups  and,  according  to  Lowsley  (Amer.  Jour.  Anat., 
vol.  13,  pp.  299-350),  number  from  53  to  74,  the  average  being  63.  In  the  male 
the  surrounding  mesenchyme  differentiates  both  white  fibrous  connective  tissue 
and  smooth  muscle  fibers  into  which  the  anlages  of  the  prostate  grow.  In  the 
female  the  tubules  remain  isolated.  The  prostatic  anlages  appear  in  male  em- 
bryos of  50  mm.  (12th  week),  chiefly  as  dorsal  and  lateral  outgrowths.  Two- 
thirds  of  the  tubules  are  caudal  to  the  openings  of  the  male  ducts.  In  the  female 
the  gland  is  rudimentary,  the  maximal  number  of  outgrowths  being  three. 

The  bulbo-urethral  glands  (of  Cowper)  arise  in  male  embryos  of  48  mm.  as 
solid  paired  epithelial  buds  from  the  entoderm  of  the  pelvic  urogenital  sinus. 
The  glands  grow  into  the  mesenchyme  which  forms  the  corpus  cavernosum  urethra, 
about  which  they  enlarge.  The  glands  branch  and,  at  120  mm.,  the  epithelium 
becomes  glandular.  The  vestibular  glands  (of  Bartholin)  are  the  homologues  in 
the  female  of  the  bulbo-urethral  glands.  They  appear  in  embryos  of  36  mm., 
grow  until  after  puberty,  and  degenerate  after  the  climacterium. 

Male  and  Female  Genitalia  Homologized. — From  the  standpoint  of  embry- 
ology the  genital  glands  are  homologous  structures.  In  the  indifferent  stage 
(Fig.  231  A),  there  are  in  both  male  and  female  a  pair  of  genital  glands,  a  pair  of 
mesonephric  or  male  ducts,  a  pair  of  Muellerian  or  female  ducts,  and  a  genital 
tubercle  bearing  the  phallus.  The  genital  ducts  open  into  the  urogenital  sinus,  a 
part  of  which  forms  the  bladder. 

Male  (Fig.  231  C). — In  the  male  the  Muellerian  ducts  degenerate  except  for 
small  portions  cranially  and  caudally,  which  persist  respectively  as  the  appendix 


236 


UROGENITAL   SYSTEM 


Fig.  231. 


Fig.  231. — Diagrams  to  show  the  devel- 
opment of  male  and  female  generating  organs 
from  a  common  type  (Allen  Thomson) . 

A.  Diagram  of  the  primitive  urogeni- 
tal organs  in  the  embryo  previous  to  sexual 
distinction:  3,  ureter;  4,  urinary  bladder;  5, 
urachus;  ot,  the  genital  ridge  from  which 
either  the  ovary  or  testicle  is  formed;  w, 
left  mesonephros;  w,  w,  right  and  left 
mesonephric  ducts;  m,  m,  right  and  left 
Miillerian  ducts  uniting  together  and  running 
with  the  mesonephric  ducts  in  g.c,  the  geni- 
tal cord;  tig,  sinus  urogenitalis;  i,  lower  part 
of  the  intestine;  cl,  cloaca;  cp,  phallus 
which  becomes  clitoris  or  penis;  Is,  fold  of 
integument  from  which  the  labia  majora 
are  formed. 

B.  Diagram  of  the  female  type  of  sex- 
ual organs:  0,  the  left  ovary;  po,  epoo- 
phoron;  w,  scattered  remains  of  mesoneph- 
ric tubules  near  it  (paroophoron);  d  G, 
remains  of  the  left  mesonephric  duct  (canal 
of  Gartner)  represented  by  dotted  lines; 
that  of  the  right  side  is  marked  w;  f,  the 
abdominal  opening  of  the  left  uterine 
tube;  u,  uterus;  the  uterine  tube  of  the 
right  side  is  marked  m;  g,  round  liga- 
ment, corresponding  to  gubernaculum;  i, 
lower  part  of  the  intestine;  va,  vagina;  h, 
situation  of  the  hymen;  C,  gland  of  Bar- 
tholin (Cowper's  gland),  and  immediately 
above  it  the  urethra;  cc,  corpus  cavernosum 
clitoridis;  sc,  vascular  bulb  or  corpus  spongi- 
osum; n,  nympha;  I,  labium;  v,  vulva. 

C.  Diagram  of  the  male  type  of  sexual 
organs:  t,  testicle  in  the  place  of  its  original 
formation;  e,  caput  epididymis;  vd,  ductus 
deferens;  W,  scattered  remains  of  the  meso- 
nephros, constituting  the  organ  of  Giralde's, 
or  the  paradidymis;  vh,  vas  aberrans;  m, 
Miillerian  duct,  the  upper  part  of  which 
remains  as  the  appendix  testis,  the  lower 
part,  represented  by  a  dotted  line  descend- 
ing to  the  prostatic  vesicle,  constitutes  the  oc- 
casionally existing  cornu  and  tube  of  the  va- 
gina masculina;  g,  the  gubernaculum;  vs,  the 
vesicula  seminalis;  pr,  the  prostate  gland;  c, 
Bulbo-urethral  gland  of  one  side;  cp,  cor- 
pora cavernosa  penis  cut  short;  sp,  corpus 
cavernosum  urethrae;  s,  scrotum;  /',  together 
with  the  dotted  lines  above,  indicates  the  di- 
rection in  which  testicles  and  epididymis  de- 
scend from  the  abdomen  into  the  scrotum. 


THE   EXTERNAL   GENITALIA  237 

testis  and  the  vagina  masculina.  The  mesonephric  duct  is  functional,  its  deriva- 
tives being  the  ductus  epididymis,  the  ductus  deferens,  the  ampulla  and  seminal 
vesicle.  The  collecting  tubules  of  the  mesonephros  form  the  ductuli  efferentia 
of  the  epididymis  and  the  vestigial  paradidymis.  The  phallus  enlarges  and  be- 
comes the  penis,  into  which  extends  a  portion  of  the  urogenital  sinus  as  the  ure- 
thra. The  genital  tubercle  disappears  and  the  scrotum  is  developed  as  a  new 
structure,  into  the  vaginal  sacs  of  which  the  testes  descend. 

Female  (Fig.  231  B). — The  genital  gland  becomes  the  ovary.  The  meso- 
nephric duct  degenerates  except  for  the  vestigial  Gartner's  canal.  Two  groups  of 
mesonephric  tubules  persist,  a  cranial  group  united  with  the  rete  ovarii  constitut- 
ing the  rudimentary  cpobphoron,  the  homologue  of  the  epididymis,  a  caudal  group 
forming  the  paroophoron,  comparable  to  the  paradidymis.  The  Muellerian  ducts 
become  the  functional  female  ducts.  Their  lower  ends  fuse  and  with  their  middle 
portions  form  the  vagina  and  uterus.  Their  upper  portions  persist  as  the 
paired  uterine  tubes.  The  phallus  remains  small  and  becomes  the  clitoris,  the 
open  lips  of  the  urethral  groove  form  the  labia  minora,  and  the  genital  tubercle 
constitutes  the  mons  veneris  and  the  paired  labia  majora  of  the  vulva.  The  ovar- 
ies descend  only  into  the  true  pelvis  but  the  lig.  ovarii  and  the  round  ligament  of 
the  uterus  are  the  homologues  of  the  gubernaculum  testis. 

Hermaphroditism. — True  hermaphroditism  consists  in  the  development  and 
persistence  of  both  testes  and  ovaries  in  the  same  individual.  It  is  of  rare  oc- 
currence in  man,  is  not  uncommon  in  the  lower  vertebrates,  and  is  the  normal 
condition  in  some  invertebrates  (earth  worms,  snails,  etc.).  In  cases  of  human 
hermaphroditism  of  this  type  the  secondary  sexual  characters  are  usually  inter- 
mediate between  the  male  and  female,  tending  now  one  way  now  the  other.  The 
external  genitalia  show  a  small  penis  with  hypospadias,  cryptorchism,  or  small 
vaginal  opening. 

False  hermaphroditism  is  characterized  by  the  presence  of  genital  glands  of 
one  sex  in  an  individual  which  exhibits  more  or  less  marked  secondary  characters 
and  external  genitalia  of  the  opposite  sex.  In  masculine  hermaphroditism  an  in- 
dividual possesses  testicles,  but  the  external  genitals  and  secondary  sexual  char- 
acters are  like  those  of  the  female.  In  feminine  hermaphroditism  ovaries  are 
present,  but  the  other  sexual  characters  are  male.  The  cause  of  hermaphroditism 
is  unknown. 


238  UROGENITAL   SYSTEM 

THE  UTERUS  DURING  MENSTRUATION  AND  PREGNANCY:  PLACENTA  AND 

DECIDUAL  MEMBRANES 

Two  sets  of  important  changes  take  place  normally  in  the  wall  of  the  uterus. 
One  of  these  is  periodic  and  is  the  cause  of  menstruation  (monthly  flow) .  These 
periodic  changes  may  also  be  regarded  as  preparatory  to  the  second  set  of  changes 
which  take  place  if  pregnancy  occurs  and  give  rise  to  the  decidual  membranes  and 
placenta. 

Menstruation. — The  periodic  changes  which  accompany  the  phenomenon  of 
menstruation  form  a  cycle  which  occupies  28  days.  This  period  is  divided  into 
(1)  a  phase  of  uterine  congestion  lasting  six  or  seven  days;  (2)  a  phase  of  hem- 
orrhage and  epithelial  desquamation,  duration  three  to  five  days;  (3)  a  phase  of 
regeneration  of  the  uterine  mucosa  lasting  four  to  six  days;  (4)  finally  an  interval 
of  rest  or  slight  regeneration,  varying  from  twelve  to  sixteen  days  duration. 

During  the  first  phase,  the  uterine  mucosa  is  thickened  to  two  or  three  times 
its  normal  condition,  both  because  of  vascular  congestion  and  on  account  of  the 
actual  increase  in  amount  of  reticular  tissue.  The  uterine  glands  become  longer 
and  their  deeper  portions  especially  are  dilated  and  more  convoluted  because 
they  are  filled  with  secretion.  From  the  enlarged  veins  and  capillaries  blood 
escapes  into  the  reticular  tissue  beneath  the  epithelium  and  forms  haematomata. 
At  the  end  of  this  phase  the  uterine  mucosa  shows  a  deep  spongy  layer  and  a 
superficial  compact  layer,  these  corresponding  to  similar  layers  in  the  decidual 
membranes  of  pregnancy. 

During  the  second  phase,  which  is  menstruation  proper,  blood  escapes  into 
the  uterine  cavity  between  the  epithelial  cells  of  the  mucosa  and  there  is  an  active 
discharge  of  secretion  from  the  uterine  glands.  The  surface  epithelium  and  a 
portion  of  the  underlying  tissue  may  or  may  not  be  desquamated.  In  some 
normal  cases  the  surface  epithelium  and  most  of  the  compact  layer  may  be  ex- 
pelled, aided  by  painful  contractions  of  the  uterus. 

In  the  third  stage,  the  mucosa  has  become  thin  with  straight  narrow  glands 
between  which  are  fusiform,  closely  packed  stroma  cells.  Any  surface  epithelium 
which  has  been  desquamated  is  regenerated  from  the  epithelium  of  the  glands  and 
gradually  the  mucosa  returns  to  a  resting  condition  during  which,  however,  there 
is  a  slow  process  of  cell  proliferation. 

The  premenstrual  changes  of  the  first  phase  are  regarded  as  the  most  impor- 
tant part  of  the  whole  process,  the  uterine  mucosa  being  prepared  for  the  reception 
of  a  fertilized  ovum  and  for  the  development  of  the  decidual  membranes.     Men- 


THE    UTERUS    DURING    MENSTRUATION    AND    PREGNANCY  239 

struation  proper,  as  seen  in  the  second  phase,  is  the  result  of  an  over-ripe  condi- 
tion of  the  mucosa  and  has  been  regarded  as  the  abortion  of  an  unfertilized  ovum. 

The  Decidual  Membranes:  Placenta 

The  Implantation  of  the  Ovum. — Our  knowledge  concerning  the  implantation  of  the 
ovum  is  fragmentary,  but  certain  facts  have  been  deduced  from  observations  on  mammals 
(hedge-hog  and  guinea-pig),  and  from  the  careful  study  of  early  human  embryos  by  Teacher, 
Bryce,  Herzog  and  Peters.  The  embryo  described  by  Teacher  and  Bryce,  while  it  is  the 
youngest  yet  observed,  is  perhaps  not  normal. 

After  ovulation,  the  ripe  ovum  is  set  free  within  the  abdominal  cavity,  from 
whence  by  the  beating  cilia  on  the  fimbrice  of  the  uterine  tube  it  is  carried  into  the 
ampulla  of  the  latter.  There  it  may  be  fertilized  and  is  swept  into  the  uterus  by 
the  cilia  of  the  tubar  epithelium.  During  this  period  of  migration,  which  is  esti- 
mated as  occupying  from  five  to  eight  days,  the  ovum  loses  its  surrounding  fol- 
licle cells  and  its  membrane  and  begins  its  development.  Thus  when  it  reaches 
the  uterus,  and  is  ready  for  implantation,  it  is  an  embryo  with  trophectoderm 
developed  but  still  not  more  than  0.2  mm.  in  diameter  (Graf  Spee). 

If  ovulation  precedes  menstruation  proper  by  ten  or  twelve  days  as  Ancel  and  Villemin 
maintain,  then  the  embryo  would  reach  the  uterus  during  the  premenstrual  period.  The  con- 
gestion and  loosening  of  the  uterine  tissue  at  this  time  would  favor  the  implantation  of  the 
embryo  and  the  glandular  secretion  would  afford  nutriment  for  its  growth  until  implantation 
occurs.  The  first  phase  of  menstruation  according  to  this  view,  that  of  Grosser,  prepares  the 
uterine  mucosa  for  the  reception  of  the  embryo.  If  pregnancy  supervenes,  it  soon  inhibits 
any  further  premenstrual  changes  so  that  menstruation  does  not  occur. 

The  embryo  penetrates  the  uterine  mucosa  as  would  a  parasite,  the  trophec- 
toderm supposedly  producing  a  ferment  which  digests  away  the  maternal  tissues 
until  the  embryo  is  entirely  embedded  (Fig.  232).  During  implantation,  the 
trophectoderm  also  probably  absorbs  nutriment  from  the  uterine  mucosa  for  the 
use  of  the  embryo.  The  process  of  implantation  is  supposed  to  occupy  one  day. 
At  the  point  where  the  embryo  enters  the  mucosa  a  fibrin  clot  soon  appears  and 
eventually  the  opening  is  completely  closed. 

The  Decidual  Membranes  (Figs.  233  and  234). — With  the  increase  in  size 
of  the  embryo  and  chorionic  vesicle,  the  superficial  layers  of  the  maternal  mucosa 
bulge  into  the  cavity  of  the  uterus  and  form  the  decidua  capsularis  fold  term  ,  de- 
cidua  reflexa) .  The  deep  layer  of  the  mucosa  on  the  side  of  the  embryo  away  from 
the  uterine  cavity  forms  the  anlage  of  the  future  maternal  placenta  and  is  the 
decidua  basalis  (d.  serotina).  The  mucosa  lining  the  rest  of  the  uterus  is  differ- 
entiated into  the  decidua  vera  ( parietal is  of  Bonnet). 

Differentiation  of  the  Trophectoderm. — The  chorion  is  at  first  composed  of 


240 


UROGENITAL   SYSTEM 


an  inner  mesodermal  layer  and  an  outer  epithelial  layer,  the  trophectoderm  (Fig. 
70) .  From  the  trophectoderm  there  is  developed  an  outer  syncytial  layer  which 
we  call  the  trophoderm  (Fig.  23  2) .  This  invades  and  destroys  the  maternal  tissues. 
In  it  large  vacuoles  are  formed  either  directly  by  the  syncytial  tissue  (Teacher  and 
Bryce)  or  by  the  blood  escaping  from  the  ruptured  vessels  under  pressure  (Peters), 
and  thus  blood  lacuna  are  produced.  The  trophoderm  thickens  at  intervals 
and  forms  on  the  surface  of  the  chorion  solid  cords  of  cells,  the  primary  villi 
(Fig.  232) .     The  chorionic  mesoderm  grows  out  into  these  cords,  the  cords  branch 


Fig.  232. — Section  through  an  embryo  of  i  mm.  embedded  in  the  uterine  mucosa  (semidiagrammatic 
after  Peters).  Am.,  amniotic  cavity;  b.c,  blood-clot;  b.s.,  body-stalk;  ect.,  embryonic  ectoderm;  era/., 
entoderm;  mes.,  mesoderm;  m.v.,  maternal  vessels;  tr.,  trophoderm;  u.e.,  uterine  epithelium;  u.g., 
uterine  glands;  y.s.,  yolk-sac. 


profusely  and  become  secondary,  or  true  villi  (Fig.  235).  During  the  development 
of  the  villi,  the  blood  lacunae  in  the  trophoderm  around  the  villi  expand,  run 
together,  and  produce  intervillous  blood  spaces  which  surround  the  villi  and  bathe 
the  epithelium  with  blood.  The  syncytial  trophoderm,  from  being  a  spongy  net- 
work, is  now  reduced  to  a  continuous  layer  covering  the  outer  surfaces  of  the  villi 
and  chorion.  Branches  of  the  umbilical  vessels  develop  in  the  mesoderm  of  the 
chorion  and  villi.  The  mesodermal  core  of  each  villus  and  its  branches  is  now 
covered  by  a  two-layered  epithelium,  an  inner  ectodermal  layer  with  distinctly 


THE   UTERUS    DURING    MENSTRUATION   AND    PREGNANCY 


241 


outlined  cubical  cells,  and  an  outer  syncytial  trophoderm  layer  (Fig.  235).  The 
epithelium  also  forms  solid  columns  of  cells  which  anchor  the  ends  of  certain  villi 
to  the  maternal  tissue.     Islands,  or  nodes  of  epithelial  cells,  are  attached  to  the 


Posterior 

lip  of  os 

uteri 


Border  cleft 


Bonier  zone 


Decidua 

basalis 


Fades 
vesicalis 

uteri 


Apex  of 
decidua 


Cervical 
canal 


Fig.  233.— Section  through  a  gravid  uterus  of  twelve  to  fourteen  days  (Kollmann's  Atlas). 


villi  or  lie  free  in  the  decidua  basalis  and  represent  portions  of  the  primitive  troph- 
ectoderm.     In  the  vessels  of  the  chorionic  villi  the  chorionic  circulation  of  the 
embryo  is  established.     The  blood-vessels  of  the  uterus  open  into  the  intervillous 
16 


242 


UROGENITAL   SYSTEM 


Fig.  234. — Diagrammatic  section  through  a  pregnant  uterus  at  the  seventh  or  eighth  week  (after 
Allen  Thomson),  c,  c,  openings  of  uterine  tubes;  c  ,  cervix  with  mucous  plug;  dv,  decidua  vera  or  pari- 
etalis;  dr,  decidua  capsularis;  ds,  decidua  basalis  (serotina);  ch,  chorion  with  villi;  the  villi  extending 
into  the  decidua  basalis  are  from  the  chorion  frondosum;  u,  umbilical  cord;  al,  allantois. 


r^-'-Jz — mcs 


Fig.  235. — Diagram  illustrating  the  second  phase  in  the  development  of  the  chorionic  villi  and 
placenta  (after  Peters),  mes,  mesoderm;  core,  core  of  villus  about  which  is  the  trophectoderm  layer; 
sy,  syncytium  of  trophoderm;   mep,  endothelium  of  maternal  capillary;  vs,  intervillous  space. 


THE    UTERUS    DURING   MENSTRUATION   AND    PREGNANCY  243 

blood  spaces  and  here  the  maternal  blood  circulates.  The  syncytial  trophoderm 
covering  the  villi  is  bathed  in  the  maternal  blood.  Its  functions  are  three-fold: 
(1)  like  endothelium  it  prevents  the  coagulation  of  the  maternal  blood;  (2)  it 
allows  of  transfusion  between  the  blood  of  fetus  and  mother;  and  (3)  it  assimi- 
lates substances  from  the  maternal  blood  and  transfers  them  to  that  of  the  embryo. 
Chorion  Laeve  and  Frondosum. — The  villi  at  first  cover  the  entire  surface  of 
the  chorion.  As  the  embryo  grows  more  and  more  out  into  the  uterine  cavity 
the  decidua  capsularis  and  that  portion  of  the  chorion  attached  to  it  are  com- 
pressed, and  the  circulation  in  the  intervillous  spaces  of  these  structures  is  cut  off 
(Figs.  234  and  236).     Thus,  beginning  at  the  pole  of  the  decidua  capsularis,  the 


Fig.  236. — Human  ova:  A,  of  three  weeks;    B,  of  six  weeks,  showing  formation  of  chorion  laeve  bj- 
degeneration  of  the  chorionic  villi  (De  Lee's  Obstetrics). 

villi  in  this  portion  of  the  chorion  degenerate  during  the  fourth  week  and  form 
the  chorion  lave.  The  villi  on  that  part  of  the  chorion  which  is  attached  to  the 
decidua  basalis  continue  their  development  and  persisting  form  the  chorion  fron- 
dosum. This,  with  the  decidua  basalis  of  the  uterus,  constitutes  the  placenta. 
The  embryo  is  attached  first  to  the  chorion  frondosum  by  the  body-stalk,  later 
by  the  umbilical  cord  (Fig.  234).  Through  the  umbilical  vein  and  arteries  in  the 
latter  the  placental  circulation  of  the  embryo  takes  place. 

The  Decidua  Vera. — During  the  first  phase  of  menstruation  the  uterine 
mucosa  begins  to  differentiate  into  a  broad  superficial  compact  layer  and  into  a 
narrower  deep  spongy  layer  in  which  are  found  the  dilated  ends  of  the  uterine 
glands.     After  pregnancy  these  two  layers  are  still  further  differentiated  in  the 


244 


UROGENITAL   SYSTEM 


wall  of  the  decidua  vera  and  d.  basalis.  The  compact  layer  is  much  thicker  than 
the  spongy  layer  and  in  it  are  found  numerous  stroma  cells,  enlarged  blood-vessels 
and  decidual  cells  (Fig.  237).  The  decidual  cells  are  derived  from  the  stroma 
cells  of  the  mucosa.  They  are  large,  being  50  /x  in  diameter,  with  clear  cytoplasm 
and  vesicular  nuclei.  Their  function  is  in  doubt.  Glycogen  has  been  found  in 
them  but  during  the  later  months  of  pregnancy  many  of  them  degenerate. 

In  the  spongy  layer  of  the  mucosa  occur  the  enlarged  and  tortuous  uterine 
glands  of  pregnancy  (Fig.  237).  During  the  first  two  months  of  pregnancy  the 
long  axes  of  the  glands  are  perpendicular  to  the  surface  of  the  mucosa.     Later,  as 


Amnion    ^^-^t^*^:^^--1' >  • 


Chorion 


aS^g^saf. ^*s$S*r ^•'T5^7>v~ 


Cavernous  layer  s*=r#i 


Muscularis  S^&s^ 


Fig.  237. — Vertical  section  through  the  wall  of  uterus  about  seven  months  pregnant  with  the  membranes 

in  situ  (Schaper  in  Lewis-Stohr).     X  30. 


the  decidua  is  stretched  and  compressed  owing  to  the  growth  of  the  fetus,  the 
glands  are  broadened  and  shortened  and  the  cavities  of  the  glands  become 
elongated  clefts  parallel  to  each  other  and  to  the  surface  of  the  decidua.  The 
gland  cells  become  stretched  and  flattened  until  they  resemble  endothelial  cells. 
At  birth,  or  in  case  of  late  abortion,  the  plane  of  separation  is  in  the  spongy  layer. 
Only  the  deep  portions  of  the  glands  remain  attached  to  the  uterine  wall  and,  by 
the  division  of  their  cells,  regenerate  the  epithelium  of  the  uterus. 

The  Decidua  Capsularis. — The  capsularis,  as  we  have  seen,  becomes  com- 
pressed as  the  embryo  grows  (Fig.  234).     To  it  is  attached  the  chorion  Iceve,  the 


THE    UTERIS    DlklNC    MENSTRUATION    AND    PREGNANCY 


245 


villi  of  which  degenerate.  With  the  increased  size  of  the  fetus,  the  capsularis 
comes  into  contact  with  the  decidua  vera  and  fuses  with  it.  Eventually  it  largely 
degenerates,  completely  so  opposite  the  internal  os  uteri,  where  the  chorionic 
villi  are  obliterated  also.  During  pregnancy,  the  lumen  of  the  cervix  is  closed  by 
a  plug  formed  by  the  secretion  of  the  glands  opening  into  the  cervix  uteri. 

The  Placenta. — The  placenta  is  composed  of  the  decidua  basalis,  constituting 
the  maternal  placenta,  and  of  the  chorion  frondosum,  the  placenta  jxtalis.  The 
area  throughout  which  the  villi  of  the  chorion  frondosum  remain  attached  to 
the  decidua  basalis  is  somewhat  circular  in  form,  so  that  at  term  the  placenta  is 
disc-shaped,  about  seven  inches  in  diameter  and  one  inch  thick  (Fig.  238).     Near 


*S 


^    ■• 


it*  0* 

*m  J 


I'll..  238. — Mature  placenta:    a,  entire  organ,  showing  fetal  surface  with  membranes  attached  to  the 
periphery;  b,  a  portion  of  attached  surface  (Heisler). 


the  middle  of  its  fetal  surface  is  attached  the  umbilical  cord,  and  this  surface  is 
formed  by  the  amnion,  the  mesoderm  of  which  is  closely  applied  to  and  fused 
with  that  of  the  chorion  frondosum  (Fig.  239). 

Chorion  Frondosum. — The  villi  of  this  portion  of  the  chorion  form  profusely 
branched  tree-like  structures  which  lie  in  the  intervillous  spaces  (Fig.  240).  The 
ends  of  some  of  the  villi  are  attached  to  the  wall  of  the  decidua  basalis  and  are 
known  as  the  anchoring  villi.  In  the  connective  tissue  core  of  each  villus  are 
usually  two  arteries  and  two  veins,  branches  of  the  umbilical  vessels,  cells  like 
lymphocytes  and  special  cells  of  Hofbauer,  the  significance  of  which  is  not  known. 
Lymphatics  are  also  present.     The  epithelium  of  the  villi,  as  we  have  seen,  is  at 


246 


UROGENITAL   SYSTEM 


first  composed  of  a  layer  of  trophectoderm  (of  Langhans)  with  the  outlines  of  its 
cuboidal  cells  sharply  defined  (Fig.  241  A).  This  layer  forms  and  is  covered  by 
a  syncytium,  the  trophoderm.     In  the  later  months  of  pregnancy  as  the  villi 


Wimm 


^^^^^w^^-^^ 


Fig.  239. — Section  through  a  normal  placenta  of  seven  months  in  situ  (Minot):  Am,  amnion;  Cho, 
chorion;  Vi,  root  of  villus;  vi,  sections  of  small  villi  ramifying  in  the  intervillous  blood  spaces;  D,  deep 
spongy  layer  showing  remnants  of  large  flattened  glands;  Ve,  uterine  vessel  opening  into  intervillous 
spaces;   Mc,  muscular  wall  of  uterus. 


THE   UTERUS   DURING    MENSTRUATION    AND    PREGNANCY 


247 


grow,  the  trophectoderm  is  used  up  in  forming  the  syncytium,  so  that  at  term  the 
trophoderm  is  the  only  continuous  epithelial  layer  of  the  villi  (Fig.  241  B).  About 
the  margin  of  the  placenta  the  trophectoderm  persists  as  the  closing  ring,  which  is 
continuous  with  the  epithelium  of  the  chorion  lasve.  Syncytial  giant  cells  found 
in  the  decidua  basalis  are  said  to  be  derived  from  the  trophoderm  of  the  villi, 


Muscular  is 


Uterine 
artery         Uterine  vein 


Uterine. 

artery  in 

septum 


Decidua 
basalis 
Uterine  artery  in 
decidual  septum 


^Intervillous 
space 


Syncytium 


Fig.    240. — Scheme  of  placental  circulation  (Kollmann's  Handatlas).       Arrows  indicate  supply  and 
exhaust  of  blood  in  the  intervillous  spaces. 


also  a  portion  of  the  canalized  fibrin  found  in  the  decidua  basalis  of  the  placenta 
near  term. 

Decidua  Basalis. — This,  the  maternal  placenta,  like  the  decidua  vera  is  dif- 
ferentiated into  a  compact  layer  or  basal  plate  which  forms  the  floor  of  the  inter- 
villous spaces,  and  into  a  deep  spongy  layer  (Figs.  239  and  240).  The  first  is  the 
remains  of  the  compact  layer  of  the  uterine  mucosa  formed  during  the  premenstrual 
phase  and  partially  destroyed  by  the  implantation  of  the  ovum.  The  second  is  the 
modified  spongy  layer  of  the  premenstrual  period  and.  though  thinner,  shows  the 
same  differentiation  as  does  this  same  layer  in  the  decidua  vera.  The  glandular 
spaces  are  less  numerous  in  the  spongy  layer  of  the  decidua  basalis  and  between 


248 


UROGENITAL   SYSTEM 


the  spaces  occur  the  syncytial  giant  cells  mentioned  above.     It  is  in  the  plane  of 
this  layer  that  the  separation  of  the  placenta  takes  place  at  birth. 

The  basal  plate,  or  compact  layer  of  the  decidua  basalis,  is  composed  of  a  con- 


Syncytium 


Cuboidal  cells  of  the  basal 
layer 


Connective  tissue 


Blood-vessel  containing 
nucleated  red  corpuscles 


Epithelium  ■ 
Epithelial  nucleus- 

Capillaries  ■*—- 


Syncytial  knot 
Small  artery  — 


Oblique  section  of  the  epithelium 


Syncytial  knot 


Epithelium 


Small  vein 


Capillary 


Syncytial  knot  — 


Fh,.  241. — Transverse  sections  of  chorionic  villi;  A ,  at  the  fourth  week;  B,  C,  at  the  end  of  pregnancy 

(Schaper  in  Stohr-Lewis). 


nective  tissue  stroma  containing  decidual  cells,  canalized  fibrin  and  persisting 
portions  of  the  epithelium  of  the  villi.  The  canalized  fibrin  is  believed  to  be 
formed  both  from  the  syncytial  trophoderm  of  the  villi  and  from  the  modified 


I  ferine  muscle 

Remaiiu 

bilical  visit. le 


Fetal  villi  of 
chorion 


Maternal  Hood 
sinus. 


Decidua 

it  septum 


Peripheral  vein 


Fused  decidua  vera 

and 


Fig.  242. — Semidiagrammatic  section  of  uterus,  showing  relations  of  fetal  and  maternal  placenta  1  Ahlfeld). 
Decidua  serolina,  decidua  basalis;  (/.  reflexa,  old  terminology  for  </.  capsularis. 


THE    UTERUS   DURING   MENSTRUATION  AND   PREGNANCY  249 

fibrin  of  the  maternal  blood.  From  the  basal  plate  septa  extend  into  the  inter- 
villous spaces  but  do  not  unite  with  the  chorion  frondosum  (Grosser  in  Keibel 
and  Mall,  vol.  i,  p.  162).  Near  term  these  constitute  the  septa  placenta  which 
incompletely  divide  the  placenta  into  lobules,  or  cotyledons  (Fig.  240).  The 
maternal  arteries  and  veins  pass  through  the  basal  plate,  taking  a  sinuous  course 
and  opening  into  the  intervillous  spaces.  Near  their  entrance  they  course  ob- 
liquely and  lose  all  but  their  endothelial  layers.  The  original  openings  of  the 
vessels  into  the  intervillous  spaces  were  formed  during  the  implantation  of  the 
ovum,  when  their  walls  were  eroded  by  the  invading  trophoderm  of  the  villi. 
As  the  placenta  increases  in  size  the  vessels  grow  larger.  The  ends  of  the  villi  are 
frequently  sucked  into  the  veins  and  interfere  with  the  placental  circulation. 
At  the  periphery  of  the  placenta  is  an  enlarged  intervillous  space,  which  varies  in 
extent  and  never  more  than  partly  surrounds  the  placenta.  This  space  is  the 
marginal  sinus  through  which  blood  is  carried  away  from  the  placenta  by  the 
maternal  veins  (Fig.  240).  The  blood  of  the  mother  and  fetus  does  not  mix, 
but  the  epithelial  cells  of  the  villi  are  instrumental  in  transferring  nutritive  sub- 
stances to  the  blood  of  the  fetus,  and  in  taking  up  excreta  from  the  fetal  circula- 
tion and  passing  them  into  the  maternal  blood  stream  of  the  intervillous  spaces. 
The  Relation  of  the  Fetus  to  the  Placenta  and  the  Separation  of  the  De- 
cidual Membranes  at  Birth. — The  relation  of  the  embryo  to  the  fetal  membranes 
has  been  described  on  p.  80.  During  the  first  months  of  pregnancy  the  embryo 
floats  in  the  cavity  of  the  amnion  attached  to  the  placenta  by  the  umbilical  cord 
(Fig.  234).  Later,  as  we  have  seen,  the  amnion  fuses  more  or  less  completely  to 
the  chorion  frondosum  and  loeve.  The  decidua  capsularis  largely  disappears  or 
is  fused  to  the  decidua  vera.  Before  birth,  the  placenta  is  concave  on  its  amniotic 
surface,  its  curvature  corresponding  to  that  of  the  uterus  (Fig.  242).  At  term, 
the  duration  of  which  is  taken  as  ten  lunar  months,  the  muscular  contractions  of 
the  uterus,  termed  "pains,  "  bring  about  a  dilation  of  the  cervix  uteri,  the  rupture 
of  the  amnion  and  chorion  lieve,  and  cause  the  extrusion  of  the  child.  With  the 
rupture  of  the  membranes  the  amniotic  liquor  is  expelled,  the  fetal  membranes 
remaining  attached  to  the  decidual  membranes.  The  pains  of  labor  begin  the 
detachment  of  the  decidual  membranes,  the  plane  of  their  separation  lying  in 
the  spongy  layer  of  the  decidua  basalis  and  decidua  vera,  where  there  are  only  thin- 
walled  partitions  between  the  enlarged  glands.  Following  the  birth  of  the  child, 
the  tension  of  the  umbilical  cord  and  the  "after  pains"  which  diminish  the  size 
of  the  uterus,  normally  complete  the  separation  of  the  decidual  membranes  from 
the  wall  of  the  uterus.     The  uterine  contractions  serve  also  to  diminish  the  size 


250  UROGENITAL   SYSTEM 

of  the  ruptured  placental  vessels  and  prevent  extensive  hemorrhage.  From  the 
persisting  portions  of  the  spongy  layer  and  from  the  epithelium  of  the  glands  the 
tunica  propria,  glands,  and  epithelium  of  the  uterine  mucosa  are  regenerated. 

The  decidual  membranes  and  the  structures  attached  to  them  when  expelled 
constitute  the  "  after-birth."  The  placenta  usually  is  everted  so  that  its  amniotic 
surface  is  convex,  its  maternal  surface  concave.  It  is  composed  of  the  amnion, 
chorion  frondosum,  villi  with  intervillous  spaces  incompletely  divided  by  the  septa 
into  cotyledons,  and  bounded  on  the  maternal  side  by  the  basal  plate  and  a  por- 
tion of  the  spongy  layer  of  the  decidua  basalis.  The  amnion  is  usually  attached 
to  the  chorion  but  may  be  free  and,  failing  to  rupture,  surround  the  child  at 
birth  as  the  "caul."  Near  the  center  of  the  placenta  is  attached  the  umbilical 
cord,  and  at  its  margins  the  placenta  is  continuous  with  the  decidua  vera  and 
the  remains  of  the  chorion  lasve  and  decidua  capsularis. 

The  Position  of  the  Placenta  in  Utero  and  its  Variations. — The  position  of 
the  placenta  is  determined  by  the  point  at  which  embryo  is  implanted.  In 
most  cases  it  is  situated  on  either  the  dorsal  or  ventral  wall  of  the  uterus.  Oc- 
casionally it  is  lateral  in  position  and  very  rarely  (1  in  1600  cases)  it  is  located 
near  the  cervix  and  covers  the  internal  os  uteri,  constituting  a  placenta  previa. 
A  partially  or  wholly  duplicated  placenta  may  be  formed  by  the  development  of 
two  groups  of  villi.  Cases  have  been  observed  in  which  from  three  to  seven  sub- 
divisions of  the  placenta  occurred. 


CHAPTER  IX 

THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 

I.  Primitive  Blood-Vessels  and  Blood-Cells 

The  blood-cells  and  primitive  blood-vessels  arise  from  a  tissue  termed  by  His 
the  angioblast.  Its  origin  has  been  in  doubt.  According  to  Minot  (in  Keibel 
and  Mall,  vol.  2) ,  it  arises  in  the  wall  of  the  yolk-sac  from  the  endoderm  and  from 
it  endothelial  sprouts  grow  into  the  body  of  the  embryo.  Another  view  as  to  its 
origin,  more  recently  championed  by  Maximow,  Felix,  Schulte  and  Bremer,  is 
that  the  angioblast  arises  from  the  mesoderm.  The  angioblast  consists  of  a  net- 
work of  solid  cords  of  cells  which  appear  first  in  the  splanchnic  mesoderm  of  the 
chorion  and  yolk-sac.  In  human  embryos  with  a  medullary  plate  about  1  mm. 
in  length,  Bremer  (Anat.  Record,  vol.  8,  p.  97,  1914)  finds  a  network  of  angio- 
blast in  the  chorion,  chorionic  villi  and  body-stalk.  This  chorionic  angioblast 
antedates  in  one  case  that  developed  in  the  yolk-sac  and  thus  must  develop  in- 
dependently from  the  splanchnic  mesoderm.  The  solid  cords  of  angioblast  soon 
hollow  out,  the  peripheral  cells  forming  the  endothelium  of  the  primitive  vessels, 
the  inner  cells  persisting  as  the  primitive  blood-cells  or  mesamoeboids  of  Minot. 
By  the  union  of  the  isolated  vascular  spaces,  the  cellular  network  is  soon  converted 
into  a  vascular  network.  In  the  wall  of  the  yolk-sac  this  network  forms  the  area 
vascidosa  (Fig.  74),  in  which  aggregations  of  blood-cells  form  the  blood-islands. 

THE  PRIMITIVE  BLOOD-CELLS  OR  MESAMCEBOIDS  (MINOT) 
These  show  large  vesicular  nuclei  surrounded  by  a  small  amount  of  finely 
granular  cytoplasm  (Fig.  243  a).  They  are  without  a  cell  membrane  and  are 
assumed  to  be  amoeboid.  During  embryonic  life,  the  mesamceboid  cells  multi- 
ply rapidly  by  mitosis  and  develop  successively  in  the  wall  of  the  yolk-sac,  in  the 
liver,  in  the  lymphoid  organs  and  in  the  red  bone  marrow. 

Minot  (in  Keibel  and  Mall)  and  many  embryologists  hold  at  the  present  time  that  the 
blood-forming  cells  of  the  adult  are  derived  directly  from  the  mesamceboid  cells  of  the  embryo. 
Maximow  (Archiv.  f.  mikr.  Anat.,  vols.  67  and  73,  pp.  680-757,  and  444-561)  maintains  that 
blood-forming  cells  may  take  their  origin  from  the  mesodermal  cells  in  embryos  and  also  from 
mesenchymal  cells  of  the  adult  connective  tissue.  From  what  is  now  known  of  the  origin  of 
the  primitive  blood-cells  Maximow's  view  seems  to  be  the  more  plausible  of  the  two. 

251 


252 


THE   DEVELOPMENT   OF   THE   VASCULAR   SYSTEM 


Origin  of  the  Erythrocytes  (Red  Blood  Corpuscles). — These  take  their  origin 
from  the  mesamceboid  cells  of  the  embryo  and  from  the  premyelocytes  of  adult 
connective  tissue  and  bone  marrow  as  erythroblasts. 

i.  Erythroblasts  (ichthyoid  blood-cells  of  Minot,  so-called  because  they  are 


Fig.  243. — Blood-cells  from  embryos  of  12  and  20  mm.  X  1160.  a.  primitive  mesamceboid  cells; 
b,  ichthyoid  cells  or  erythroblasts;  c,  sauroid  cells;  d,  sauroid  cells;  e,  cup-shaped  nucleated  cells;  /, 
erythrocytes,     a,  b  and  c  are  from  a  12  mm.  human  embryo;  d,  e,  and/,  from  a  20  mm.  embryo. 


the  typical  red  blood-cells  of  fishes),  are  characterized  by  the  presence  of  hemo- 
globin in  the  homogeneous  cytoplasm,  which  is  thus  colored  red.  The  nuclei 
are  vesicular  with  granular  chromatin  (Fig.   243  b).      There  is  a  definite  cell 

membrane.  The  erythroblasts  are  the  only 
red  blood-cells  of  the  first  month  of  em- 
bryonic development,  occurring  in  embryos 
of  10  mm. 

2.  Normoblasts,  termed  sauroid  blood- 
cells  because  they  are  the  red  blood-cells  of 
adult  reptiles,  are  first  formed  in  the  liver  from 
the  erythroblasts  in  embryos  of  the  second 
month,  and  are  predominant  at  this  stage. 
They  are  distinguished  by  their  small  round 
nuclei  with  dense  chromatin  which  stains  so 
heavily  that  little  or  no  structure  can  be  seen  (Fig.  243  c,  d).  The  cytoplasm  is 
larger  in  amount  than  in  erythroblasts. 

According  to  Maximow,  the  primitive  erythroblasts  in  rabbit  embryos  all  degenerate  and 
the  normoblasts  are  developed  as  a  new  generation  of  cells  from  primitive  lymphocytes  (mes- 
amceboids  of  Minot). 


Fig.  244. — The  development  of 
red  corpuscles  in  cat  embryos  (How- 
ell), a,  successive  stages  in  the  de- 
velopment of  a  normoblast;  b,  the 
extrusion  of  the  nucleus. 


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Fig.  245.— Human  blood  cells,  r-:2Z;  cells  from  the  red  bone  marrow  of  the  mouse,  22-31  (Sobotta). 
6-12,  erythrocytes,  9  showing  a  nucleus;  /,  lymphocyte;  2,  3,  4,  5,  74,  rtf,  rS,  neutrophilic  polymorpho- 
nuclear leucocytes;  15,  19,  21,  eosinophiles;  13,  17,  20,  mononuclear  leukocytes;  22,  giant  marrow  cell; 
23,  24,  neutrophils  of  marrow;  23,  26,  eosinophiles  of  marrow;  27,  28,  cells  in  mitosis;  29,  erythrocyte; 
30,  31,  erythroblasts  (X  700)  • 


THE   PRIMITIVE    BLOOD-CELLS   OR   MESAMCEBOIDS  253 

3.  Erythrocytes  (red  blood  corpuscles,  blood  plastids)  (Minot)  are  developed 
in  mammals  from  normoblasts  which  lose  their  nuclei  by  extrusion  (Fig.  243  F). 
The  nucleus  may  be  extruded  as  several  small  granules  or  as  a  whole  (Fig.  244). 

Emmcl  (Amer.  Jour.  Anal.,  vol.  16)  has  studied  cultures  of  blood-cells  from  pig  em- 
bryos and  has  observed  the  formation  of  bodies  resembling  erythrocytes  by  the  fission  of  the 
cytoplasm.  He  suggests  that  this  may  be  their  normal  method  of  development  in  the  em- 
bryo. 

The  first  red  blood  corpuscles  are  spherical  and  are  formed  during  the  second 
month  chiefly  in  the  liver.  During  the  third  month  the  enucleated  erythrocytes 
predominate  and  are  disc-like  or  cup-shaped  in  form  (Fig.  243  /) .  During  the  later 
months  of  fetal  life,  the  red  blood  corpuscles  are  developed  in  the  liver,  in  the  red 
bone  marrow  and  possibly  in  the  spleen.  According  to  the  view  of  Minot,  the 
cells  from  which  they  take  their  origin  are  mesamceboids  which  have  lodged  in 
the  blood-forming  organs  and  undergo  cell  division  and  differentiation  there.  In 
the  bone-marrow  these  cells  are  known  as  premyelocytes.  They  differentiate  into 
both  crylhroblasts  and  myelocytes;  from  the  former  normoblasts  and  erythro- 
cytes arise;  from  the  myelocytes  the  granular  leucocytes  are  developed.  Soon 
after  birth  the  red  bone  marrow  is  the  only  source  of  new  red  blood  corpuscles. 

Origin  of  the  Leucocytes,  or  white  blood-cells  (Fig.  245). — These  are  divided 
into  non-granular  and  granular  types.  It  is  assumed  that  both  types  are  derived 
from  the  primitive  mesamccboid  cells  of  the  embryo. 

I.  Non-granular  Leucocytes. 

1.  Small  lymphocytes  (22  to  25  per  cent,  of  the  leucocytes  in  adult  blood)  are 
regarded  as  young  leucocytes.  They  vary  from  4  to  7.5  fx  in  diameter  and  are 
developed  in  the  lymphoid  organs  of  the  embryo  and  adult.  The  large  nuclei 
containing  several  connected  masses  of  chromatin  stain  darkly  and  are  surrounded 
by  a  narrow  zone  of  clear  basic  cytoplasm. 

2.  Large  mononuclear  leucocytes  (1  to  3  per  cent,  of  leucocytes)  are  developed 
from  the  endothelial  cells  lining  the  medullary  sinuses  of  the  lymph  glands.  This 
may  be  demonstrated  by  intra  vitam  staining  with  trypan  blue  (Evans,  Anat. 
Record,  vol.  8,  p.  99,  1914). 

II.  Granular  or  Polymorphonuclear  Leucocytes. 

The  blood-forming  cells  lodged  in  the  red  bone  marrow  are  known  as  pre- 
myclocytes. They  give  rise  to  myelocytes,  cells  with  round  or  crescentic  nuclei  and 
granular  cytoplasm.  Similar  cells  are  developed  in  the  lymphoid  organs.  By 
undergoing  changes  (1)  in  the  form  and  structure  of  their  nuclei,  (2)  in  the  size 


254  THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 

and  staining  qualities  of  their  cytoplasmic  granules,  the  myelocytes  give  rise  to 
three  types  of  granular  leucocytes: 

i.  Nentr  ophites,  or  leucocytes  with  a  finely  granular  cytoplasm  which  is 
neutral  in  its  staining  reactions,  coloring  slightly  with  both  acid  and  basic  stains. 
In  development,  their  nuclei  take  up  an  eccentric  position  and  become  crescentic, 
horse-shoe  shaped,  or  in  the  older  stages  moniliform  (three  or  four  pieces  linked 
together).  As  it  changes  in  form  the  nucleus  undergoes  pyknosis  and  stains  in- 
tensely. Neutrophiles  are  produced  in  the  bone  marrow  of  the  embryo  during 
the  fifth  month.  In  the  human  adult  they  form  70  to  72  per  cent,  of  the  leuco- 
cytes in  normal  circulation. 

2.  Eosinophiles,  or  coarsely  granular  leucocytes,  are  characterized  by  their 

large  cytoplasmic  granules 
which  stain  intensely  red 
with  eosin.  In  development 
the  nucleus  becomes  bilobed. 
Eosinophiles  form  2  to  4  per 
cent,  of  the  leucocytes  in 
normal  human  blood. 


jjfij:  According   to   Weidenreich 

(Arch.  f.  mikr.  Anat.,  vol.  82,  pp. 
282-286),  the  eosinophilic   gran- 
ules are  not  endogenous  but  are 
fragments  of  red  blood  corpuscles 
Fig.  246.— Giant  cell  from  the  bone  marrow  of  a  kitten,         which  have  been  ingested  by  the 
showing  pseudopodia  extending  into  a  blood-vessel  (V),  and         leucocvtes     or  are   formed    from 
giving  rise  to  blood-plates  (bp)  (J.  H.  Wright).  hemoglobin  derivatives.     Bader- 

scher  (Amer.  Jour.  Anat.,  1913, 
vol.  15,  pp.  69-86)  finds  in  the  vicinity  of  degenerating  muscle  fibers  in  salamanders  numer- 
ous eosinophiles.  Also  during  trichiniasis  in  man,  when  there  is  extensive  degeneration  of 
muscle  fibers,  the  number  of  eosinophiles  in  the  blood  becomes  greatly  increased.  Downey 
{Anat.  Record,  vol.  8,  p.  135,  1914)  finds  that  the  granules  of  eosinophilic  myelocytes  dif- 
ferentiate from  a  non-granular  cytoplasm.     These  basophilic  granules  become  eosinophilic. 

3.  Basophiles,  or  Mast  Leucocytes  (Maximow),  form  only  0.5  per  cent,  of  the 
leucocytes.  Their  nuclei  are  very  irregular  in  form  and  may  be  broken  down  into 
several  pieces  which  stain  intensely.  The  granules  are  variable  in  number,  size 
and  form,  and  often  stain  so  heavily  as  to  obscure  the  nucleus.  The  cytoplasm 
is  clear  and  vacuolated.  Basophiles  have  been  regarded  as  degenerating  granular 
leucocytes  but  at  present  this  view  is  not  generally  accepted. 

Origin  of  the  Blood  Plates. — In  the  bone  marrow  and  spleen  pulp  are  giant 


EARLY   DEVELOPMENT   OF    THE   HEART  AND    PAIRED   BLOOD-VESSELS         255 

cells,  the  cytoplasm  of  which  shows  a  darkly  staining  granular  endoplasm  and  a 
clear  hyaline  exoplasm  (Fig.  246).  According  to  Wright  (Jour.  Morphol., 
vol.  21,  pp.  265-278),  the  blood  plates  arise  by  being  pinched  off  from  cytoplasmic 
processes  of  the  giant  cells.  Wright  has  shown  that  genuine  blood  plates  and 
giant  cells  occur  only  in  mammals. 

The  granules  of  the  plates  are  interpreted  by  Wright  as  derived  from  the  endoplasm  of 
the  giant  cells  and„stain  in  a  similar  manner.  This  view  has  been  generally  accepted  by  Ameri- 
can embryologists  who  have  seen  Wright's  preparations.  Schafer  regards  the  blood  plates  as 
minute  cells,  and  the  granular  endoplasm  of  Wright  as  a  small  nucleus. 

EARLY  DEVELOPMENT  OF  THE  HEART  AND  PAIRED  BLOOD-VESSELS 
We  have  seen  that  the  first  blood-cells  and  blood-vessels  take  their  origin  in 
the  angioblast,  which  develops  in  the  wall  of  the  yolk-sac  and  chorion  probably 
from  the  splanchnic  mesoderm.  The  first  vessels  derived  from  the  angioblast 
(see  p.  251)  are  small  isolated  blood  spaces  which  unite  and  form  capillary  net- 
works. From  these  endothelial  sprouts  grow  out,  meet  and  unite  until  complete 
networks  are  formed.  In  human  embryos  of  1  mm.  or  less  these  envelop  the  lower 
portion  of  the  yolk-sac,  the  body-stalk  and  chorion.  The  origin  of  the  heart  and 
paired  vascular  trunks  of  human  embryos  is  in  doubt,  but  some  facts  are  certain 
from  our  study  of  their  development  in  birds  and  mammals. 

According  to  His  and  Minot,  all  the  blood-vessels  of  the  embryonic  body  arise  as  endo- 
thelial ingrowths  from  the  primitive  vascular  area  of  the  yolk-sac.  According  to  the  investi- 
gations of  Mollier  (Hertwig's  Handb.,  vol.  i,  1906),  in  all  vertebrates  the  endothelial  anlage 
of  the  heart  is  represented  by  cells  which  appear  independent  of  the  vascular  area  between  the 
entoderm  and  the  mesoderm  in  the  distal  portion  of  the  head.  These  vascular  cells  occur 
as  paired  anlages.  According  to  Mollier,  vascular  anlages  arise  in  situ  and  give  rise  to  the  endo- 
thelium of  the  heart.     Similarly  other  vascular  anlages  form  the  primitive  aortic  trunks. 

Evans  (Amer.  Jour.  Anat.,  vol.  ix,  1909)  by  injecting  young  chick  em- 
bryos has  shown  conclusively  that  most  of  the  descending  aorta  "is  formed  from 
the  medial  margin  of  the  vitelline  capillary  plexus."  In  mammals,  connection 
of  the  same  plexus  with  the  descending  aorta  has  been  demonstrated  by  Turs- 
ting.  Bremer  {Amer.  Jour.  Anat.,  vol.  13,  1912,  pp.  111-128)  in  summarizing 
his  work  on  the  development  of  the  aorta  and  aortic  arches  in  the  rabbit  says: 
"The  dorsal  aorta,  the  first  aortic  arch,  the  conus  arteriosus,  and  the  lateral  heart 
are  all  parts  of  an  original  network  of  angioblast  cords  derived  from  the  extra- 
embryonic plexus  of  blood-vessels." 

Bremer  points  out  that  as  the  true  vessels  with  cavities  develop  they  are  connected  by 
intervening  cords  of  the  angioblast.     This  connection  cannot  be  demonstrated  by  injection 


256 


THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 


methods.  He  believes  that  Mollier  and  Tursting  have  overlooked  the  angioblast  cords  between 
the  capillary  spaces  and  have  thus  described  them  as  vascular  anlages  independent  of  the 
extraembryonic  plexus. 

It  thus  seems  probable  that  the  endothelium  of  the  primitive  heart  and  ves- 
sels has  a  common  origin  from  the  endothelial  cells  of  the  area  vasculosa.  After 
the  development  of  the  endocardium  and  primitive  aortae  it  is  certain  that  most 

other  vascular  trunks  are  formed 
first  as  capillary  plexuses.  By  en- 
largement and  differentiation  of 
definite  paths  in  such  a  capillary 
plexus  the  arterial  and  venous 
trunks  are  developed.  By  the  in- 
jection methods  of  Mall  and  his 
students  such  capillary  plexuses 
have  been  demonstrated  in  the 
limb  buds,  in  the  head  and  in 
many  organs  of  chick  and  pig  em- 
bryos (Fig.  247) .  Exceptions  to  the 
general  rule  are  the  intersegmental 
arteries  which  arise  as  single  trunks 
from  the  aorta  (Evans  in  Keibel  and 
Mall,  vol.  2). 

Origin  of  the  Tubular  Heart. — 
In  chick  and  mammalian  embryos 
it  is  known  that  paired  endothelial 
anlages  of  the  heart  bulge  into  a 
fold  of  the  splanchnic  mesoderm 
on  each  side  when  the  embryo  is 
still  flattened  on  the  surface  of  the 
yolk  (Fig.  248,  A).  Paired  endothelial  anlages  are  present  in  the  Spee  human 
embryo  1.54  mm.  long.  As  the  embryo  grows  away  from  the  yolk,  and  the  head- 
fold  elongates,  the  entoderm  is  withdrawn  from  between  the  endothelial  anlages 
and  these  at  once  fuse  (Tig.  248,  B,  C).  The  heart  is  now  an  unpaired  endothelial 
tube  lying  in  the  folds  of  the  splanchnic  mesoderm.  Soon  the  ventral  attachment 
of  the  mesoderm  disappears,  leaving  the  heart  suspended  by  the  dorsal  meso- 
cardium  in  the  single  pericardial  chamber  (Fig.  248,  C).  The  endothelial  tube 
forms  the  endocardium,  the  splanchnic  mesoderm  later  gives  rise  to  the  epicardium 


Fig.  247. — The  caudal  end  of  a  chick  embryo  of 
32  somites,  showing  the  primary  capillary  plexus 
in  the  posterior  limb  buds.  26th  Dor.  Seg.  Vein, 
twenty-sixth  dorsal  segmental  vein,  i.  e.,  that  in  the 
twenty-seventh  interspace  (Evans). 


EARLY   DEVELOPMENT   OF    THE   HEART   AND   PAIRED   BLOOD-VESSELS 


257 


and  myocardium  (muscle  layer  of  heart).  This  type  of  heart  occurs  in  human  em- 
bryos of  2  mm.  and  5  and  6  somites  (Fig.  249)  and  shows  three  regions:  (1)  the 
atrium,  which  receives  the  blood  from  the  primitive  veins,  (2)  the  ventricle,  (3) 
the  bulb,  from  which  is  given  off  the  ventral  aorta. 

As  the  cardiac  tube  grows  faster  than  the  pericardial  cavity  in  which  it  lies 

it  bends  to  the  right,  the  bulbus 
and  ventricle  forming  a  U-shaped 
loop  (Fig.  250,  A,  B).  Four  regions 
may  now  be  distinguished:  (1)  the 
sinus  venosus;  (2)  the  atrium,  also 
thin  walled  and  lying  cranial  to 
the  sinus;  (3)  the  thick-walled  ven- 
tricular limb,  ventrad  and  caudad  in 
position;  (4)  the  bulbar  limb,  cranial 


^-  en, 

Fig.  .24S. — Diagrams  to  illustrate  the  origin  of  the 
tubular  heart  (Strahl  and  Carius,  from  McMurrich's 
"  Development  of  Human  Body"),  am,  amnion;  en, 
entoderm;  //.  heart;   /,  digestive  tract. 


Fig.  249. — The  heart  of  a  2  mm.  human  em- 
bryo in  ventral  view  (Mall). 


to  the  ventricular  limb  and  separated  from  it  by  the  bulbo-ventricular  cleft. 
Xext  in  embryos  of  3  to  4  mm.  the  bulbo-ventricular  loop  shifts  its  position  until 
its  base  is  directed  caudad  and  ventrad  (Fig.  250,  B).  At  the  same  time  the  sinus 
venosus  is  brought  dorsal  to  the  atrium,  which  in  turn  is  cranial  with  relation 
to  the  bulbo-ventricular  loop,  and  the  bulbar  limb  is  pressed  against  the  ventral 
surface  of  the  atrium  and  constricts  it. 
17 


258 


THE   DEVELOPMENT   OF   THE   VASCULAR   SYSTEM 


In  embryos  of  4  to  5  mm.  the  right  portion  of  the  sinus  venosus  grows  more 
rapidly  than  the  left,  this  being  due  to  the  fact  that  the  blood  flow  of  the  left 
umbilical  vein  is  shifted  to  the  right  side  through  the  liver.  As  a  result  the  en- 
larged right  horn  of  the  sinus  opens  into  the  right  dorsal  wall  of  the  atrium  through 
a  longitudinally  oval  foramen,  which  is  guarded  on  the  right  by  a  vertical  fold. 

A  B 


Fig.  250. — A,  heart  of  human  embryo  of  2.15  mm.  (His):  a,  bulbus  cordis;  b,  primitive  ventricle; 
c,  atrial  portion.  B,  heart  of  human  embryo  of  about  3  mm.  (His):  a,  bulbus  cordis;  b,  atrial  portion 
(behind);   c,  primitive  ventricle  (in  front). 


Fig.  251. — A,  heart  of  human  embryo  of  about  4.3  mm.  (His):  a,  atrium;  b,  portion  of  atrium 
corresponding  with  auricular  appendage;  c,  bulbus  cordis;  d,  atrial  canal;  e,  primitive  ventricle.  B, 
heart  of  human  embryo  of  about  the  fifth  week  (His):  a,  left  atrium;  b,  right  atrium;  c,  bulbus  cordis; 
d,  interventricular  groove;  e,  right  ventricle;  /,  left  ventricle. 


This  fold,  which  projects  into  the  atrium,  is  the  right  valve  of  the  sinus  venosus. 
Later,  a  smaller  fold  forms  the  left  valve  of  the  sinus  venosus  (Fig.  253,  B).  The 
atrium  is  constricted  dorsally  by  the  gut,  ventrad  by  the  bulbus.  It  therefore 
must  enlarge  laterally  and  in  so  doing  forms  the  right  and  left  atria  (Fig.  251,  A, 
B)  with  the  distal  portion  of  the  bulb  between  them.     The  deep  external  groove 


EARLY   DEVELOPMENT    OP    THE   HEART   AM)    PAIRED    JiLOOD-VESSELS 


259 


between  the  atria  and  the  bulbo-ventlicular  part  of  the  heart  'is  the  coronary 
sulcus.  As  the  bulbo-ventricular  region  increases  in  size,  the  duplication  of  the 
wall  between  the  two  limbs  lags  behind  in  development  and  finally  disappears 
I  Fig.  252  (/.  b),  leaving  the  proximal  portion  of  the  bulb  and  the  ventricular  limb 
to  form  a  single  chamber,  the  primitive  ventricle.  In  an  embryo  of  5  mm.  the  heart 
is  thus  composed  of  three  undivided  chambers:  (1)  the  sinus  venosus  opening 
dorsad  into  the  right  dilation  of  the  atrium;  (2)  the  bilaterally  dilated  atrium 
opening  by  the  single  transverse  atrial  canal  into  (3)  the  primitive  undivided  ven- 
tricle. The  three-chambered  heart  is  persistent  in  adult  fishes,  but  in  birds  and 
mammals  a  four-chambered  heart  is  developed  in  which  circulates  venous  blood 
on  the  right  and  arterial  blood  on  the  left. 

The  important  changes  leading  to  the  formation  of  the  four-chambered  heart 


Fig.  252. — Reduction  of  the  bulbo-ventricular  fold  of  the  heart  (Keith).     Ao,  aortic  bulb;  An.  atrium; 
B,  bulbus  cordis;   RV,  right  ventricle;  LV,  left  ventricle;  P  (in  b)  pulmonary  artery. 


are:  (1)  the  complete  division  of  the  atrium  and  ventricle,  each  into  right  and  left 
chambers;  (2)  the  division  of  the  bulb  and  truncus  arteriosus  into  the  aorta  and 
pulmonary  artery;  (3)  the  absorption  of  the  sinus  venosus  into  the  wrall  of  the 
right  atrium ;  (4)  the  development  of  the  semilunar  and  atrio- ventricular  valves. 
The  first  of  these  changes  is  completed  only  after  birth. 

Endocardial  Cushions  and  Atrial  Septa. — In  embryos  of  5  to  7  mm.  there 
develops  a  thin  sickle-like  membrane  from  the  mid-dorsal  wall  of  the  atrium  (Figs. 
253  and  254).  This  is  called  the  atrial  septum  primum  (I).  Simultaneously, 
endothelial  thickenings  appear  in  the  dorsal  and  ventral  walls  of  the  atrial  canal 
(Fig.  254,  A,  B).  These  are  the  endocardial  cushions  which  later  fuse,  thus  divid- 
ing the  single  atrial  canal  into  right  and  left  atrio-vcntricular  canals.  The  atrium 
is  now  partly  divided  into  right  and  left  atria  which,  however,  communicate  ven- 


260 


THE   DEVELOPMENT   OF   THE   VASCULAR   SYSTEM 


trad  through  the  interatrial  foramen.  Next  in  embryos  of  9  mm.  the  septum  I 
thins  out  dorsad  and  cephalad  and  a  second  opening  appears,  the  foramen  ovale 
(Figs.  253  and  254,  B).  The  atria  are  now  connected  by  two  openings,  the  inter- 
atrial foramen  and  the  foramen  ovale.     Soon  (embryos  of  10  to  12  mm.)  the  ven- 


Valves  of 

Sinus  venosus 

Septum  I 


Mrio-ven1ricu/ar 
opening 


Valves  if  sinus  venoms  • 
•ept.I 

■Foramen  ovate 


Interventricular 
Septum 


Oinus   venos, 
B.valve sinus  venpsus 


R.  common  cardinal  vein 


Septum  E 
L.valve  Sinus    venosus 


Atrio-  ventricular 
groove 

R.vent, 


Vent. 


Fig.  253.— Oblique  transverse  sections  of  heart  wall:   A,  6  mm.;   B,  9  mm.;   C,  12  mm.    (A  and  B  are 

'  based  on  figures  of  Tandler). 


tral  and  caudal  edge  of  septum  I  fuses  with  the  endocardial  cushions,  which  have 
in  turn  united  with  each  other  (Figs.  253  and  254,  C).  The  interatrial  foramen 
is  thus  obliterated,  but  the  foramen  ovale  persists  until  after  birth.  In  embryos 
of  9  mm.  the  septum  secundum  is  developed  from  the  dorsal  and  cephalic  wall 
of  the  atrium  just  to  the  right  of  the  septum  primum  (Fig.  253,  C).     It  is  impor- 


EARLY    DEVELOPMENT    OF    THE    HEART   AND    PAIRED    BLOOD-VESSELS 


26l 


tant,  as  it  later  fuses  with  the  left  valve  of  the  sinus  venosus  and  with  it  forms  a 
great  part  of  the  atrial  septum  of  the  late  fetal  and  adult  heart. 

Sinus  Venosus  and  its  Valves. — The  opening  of  the  sinus  venosus  into  the 


3eptn 


Lva.ii/ 


Sept.l 


L.atr. 


Vend., 


L  vent 


L  atr 


■Sufritc 


L.venl 


L  .vent 


A  B  C 

Fig.  254. — Lateral  dissections  of  the  heart  from  the  left  side.  A,  6  mm.;  B,  9  mm.;  C,  12  mm. 
(A  and  B  are  based  on  reconstructions  by  Tandler).  Cor.  sin.,  coronary  sinus;  D.  end.  c,  dorsal 
endocardial  cushion;  For.  in.,  foramen  ovale;  Int.  for.,  interatrial  foramen;  /.  v.  c,  inferior  vena  cava; 
L.  air.,  left  atrium;  L.  va.  s.  v..  left  valve  of  sinus  venosus;  L.  vent.,  left  ventricle;  P11I.  a.,  pulmonary 
artery;  Pul.  ?..  pulmonary  vein;  Sept.  I.  Sept.  II,  septum  primum,  septum  secundum;  Sup.  :.  c, 
superior  vena  cava;    V.  end.  c.,  ventral  endocardial  cushion. 


loramen    ovale 

n.  valve  ofs/hi/s  Vet/ows 
Inf.  Vena  cava 


Sup.  Vena  Cava, 
•Septum  E 

'    Aortc 


Semilunar  valve 
of  pulmonary 
artery 


R. Ventricle 


Fig.  255. — Dissection  of  the  heart  of  a  65  mm.  embryo  from  the  right  side.     X  12. 


262 


THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 


dorsal  wall  of  the  right  atrium  is  guarded  by  two  valves  (Fig.  253).  Along  the 
dorsal  and  cephalic  wall  of  the  atrium  these  unite  to  form  the  septum  spurium. 
Caudally  the  valves  flatten  out  on  the  floor  of  the  atrium,  but  the  left  valve  later 
becomes  continuous  with  the  atrial  septum  II.  In  embryos  of  10  to  20  mm.  the 
atria  increase  rapidly  in  size  and  the  right  horn  of  the  sinus  venosus  is  taken  up 
into  the  wall  of  the  right  atrium.  The  superior  vena  cava  now  opens  directly 
into  the  cephalic  wall  of  the  atrium,  the  inferior  vena  cava  into  its  caudal  wall. 


Crista  lerminalis 


Jept.H 
L  valve  of 
jinus  Venosus 

Septum  I. 


Inf.  vena 
cava 

Vatve  of 

inf.  vena  cava 


up.  Vena.  Cai/frf opened) 


foramen 
ovate 


Aorta 


■SemiU 


Valve  of 
Coronary  sinus 


Tricuspid  valve 


valves  of 
jouLm.artery 


Ti.ventride 


Fig.  256. — Dissection  of  the  heart  of  a  105  mm.  fetus  from  the  right  side.     X  7. 


The  transverse  portion  of  the  sinus  venosus,  as  the  persisting  coronary  sinus, 
opens  into  the  posterior  wall  of  the  atrium. 

The  right  valve  of  the  sinus  venosus  is  very  high  in  10  to  65  mm.  embryos  (first 
to  third  month)  and  nearly  divides  the  atrium  into  two  chambers  (Fig.  255). 
It  becomes  relatively  lower  during  the  third  and  fourth  months.  Its  cephalic 
portion  becomes  the  rudimentary  crista  terminalis  (Fig.  256);  the  remainder  is 
divided  by  a  ridge  into  two  parts,  of  which  the  larger  cephalic  division  persists  as 
the  valve  of  the  inferior  vena  cava  (Eustachian  valve)  located  at  the  right  of  the 


EARLY   DEVELOPMENT   OF    TIIK    HEART   AND    PAIRED    BLOOD-VESSELS 


263 


opening  of  the  vein,  and  the  smaller  caudal  portion  becomes  the  valve  of  the  coro- 
nary sinus  (Thebesian  valve). 

The  left  valve  of  the  sinus  venosus  becomes  continuous  with  the  septum  se- 
cundum and  in  embryos  of  20  to  22  mm.  or  larger  the  two  bound  an  oval  opening 
(Figs.  257  and  258).     The  bounding  wall  of  the  oval  aperture  is  the  limbus  ovalis. 

Closure  of  the  Foramen  Ovale. — The  free  edge  of  septum  I  is,  in  embryos  of 
10  to  15  mm.,  directed  dorsad  and  cephalad  (Fig.  254,  C).  Gradually  in  later 
stages  (Figs.  257  and  258)  its  caudal  and  dorsal  prolongation  grows  cephalad  and 


/Septum  II 


dun. 


up.vena.cai/CL. 


Aorta 


Toramen    ovale 


Pulmonary  — t4gBP 
trunk  |T 


La.Tr  1  um 


••• 


Septum  I 

n  f.  vena  cava. 


oronaty  sinus 


y. 


Bi  cuspid  valve 


L. Ventricle 


Fig.  257. — Dissection  of  the  heart  of  a  65  mm.  embryo,  from  the  left  side,  showing  the  septa  and  the 

foramen  ovale.     X  8. 


ventrad  until  its  free  edge  is  so  directed.  Pari  passu  with  this  change  the  septum 
II  with  its  free  edge  directed  at  first  ventrad  and  caudad  shifts  until  its  free  edge 
is  directed  dorsad  and  cephalad,  and  overlaps  the  septum  I  (Figs.  254,  C,  257,  258). 
The  opening  between  these  septa  persists  until  after  birth  as  the  foramen  ovale. 
During  fetal  life  the  left  atrium  receives  little  blood  from  the  lungs,  so  that 
the  pressure  is  much  greater  in  the  right  atrium.  As  a  result,  the  septum  I  is 
pushed  to  the  left  and  the  blood  flows  from  the  right  into  the  left  atrium  through 
the  foramen  ovale.     After  birth  the  left  atrium  receives  from  the  expanding  lungs 


264 


THE    DEVELOPMENT    OF    THE    VASCULAR    SYSTEM 


as  much  blood  as  the  right  atrium,  the  septum  I  is  pressed  against  the  limbus  of 
septum  II,  and  soon  fuses  with  it.  The  depression  formed  by  the  thinner  walled 
septum  I  is  the  fossa  ovalis. 

The  foramen  ovale  may  fail  to  close  soon  after  birth  and  the  mixed  blood  produces  a 
purplish  hue  in  the  child  which  is  known  popularly  as  a  "blue  baby."  This  condition  may 
be  persistent  in  adult  life. 

Pulmonary  Veins. — In  embryos  of  6  to  7  mm.  a  single  vein  grows  out  from 
the  caudal  wall  of  the  left  atrium  to  the  left  of  the  septum  I.     This  vein  bifurcates 


Sup  V.6. 


Sept.! 


Pul. 


I.v.e 


Cor.sin,     l_Vent 

L.atr. 
Vent.  c. 


L.vent 


0ePtJ 


I.V.C. 


Cor.  sin. 


Bic.  va. 


A  B 

Fig.  258. — Dissections  from  the  left  side  of  human  hearts:  A ,  from  a  22  mm.  embryo;  B,  from  a  105 
mm.  embryo.  Cor.  sin.,  coronary  sinus;  For.  ov.,  foramen  ovale;  I.v.c,  inferior  vena  cava;  L.  air. 
vent,  c,  left  atrio-ventricular  canal;  L.  vent.,  left  ventricle;  Pul.  a.,  pulmonary  artery;  Sept.  I,  Sept.  II, 
septum  primum  and  septum  secundum;  Bic.  va.,  bicuspid  valve. 


into  right  and  left  pulmonary  veins  which  divide  again  before  entering  the  lungs. 
As  the  atrium  grows,  the  proximal  portion  of  the  pulmonary  vein  is  taken  up  into 
the  atrial  wall.  As  a  result,  at  first  two,  then  four,  pulmonary  veins  open  into 
the  left  atrium. 

Origin  of  the  Right  and  Left  Ventricles. — In  embryos  of  5  to  6  mm.  there 
appears  at  the  base  of  the  primitive  ventricular  cavity  a  sagittally  placed  eleva- 
tion, the  interventricular  septum  (Fig.  253,  B).  It  later  grows  cephalad  and  dorsad 
toward  the  endocardial  cushions,  and  forms  an  incomplete  partition  between  the 
right  and  left  ventricles,  which  still  communicate  through  the  persisting  inter- 


EARLY   DEVELOPMENT    OP    THE   HEART   AND    PAIRED   BLOOD-VESSELS         265 

ventricular  foramen.     Corresponding  to  the  internal  attachment  of  the  septum 

there  is  formed  externally  the  interventricular  sulcus  which  marks  the  external 

line  of  separation  between  the  large  left  ventricle  and  the  smaller  right  ventricle. 

Origin  of  Aorta  and  Pulmonary  Artery  from  Bulbus.     Coincident  with  the 


Pulmonary  artery 
Dorsal  swelling 


Ventral  endocardial 
cushion 


Aorta 


Dorsal  endocardial 
cushion 


Alrio-ventrieular 
foramen 

Interventricular 

septum 

Interventricular 
sulcus 


Aorta 


Arrow  in  pulmonary 
artery 

Prox.  bulbar  septum 


Interventricular 

foramen 

R.  atrio-vcntricular 
foramen 


R.  ventricle 


Pulmonary  artery 
Base  of  aorta 

Arro'd.'  in  aorta 


L.  atrio-vcntricular 
foramen 


I  ntcrvcntricular 
septum 


Fig.  259. — Two  stages  in  the  development  of  the  heart  to  show  the  differentiation  of  the  bulbus 
cordis  into  the  aorta  and  pulmonary  trunk:  .1,  heart  of  a  5  mm.  embryo;  B,  of  a  7.5  mm.  embryo 
(Kollmann's  Handatlas). 


266 


THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 


formation  of  the  interventricular  septum  there  arise  in  the  aortic  bulb  longitudinal 
thickenings,  four  in  the  distal  half,  two  in  the  proximal  half  of  the  bulb.  Of  the  four 
distal  thickenings  two,  which  we  will  number  a  and  c,  are  larger  than  the  other 
thickenings  b  and  d.  Thickenings  a  and  c,  which  distally  occupy  right  and  left 
positions  in  the  bulb,  meet,  fuse  and  divide  the  bulb  into  a  dorsally  placed  aorta 
and  ventrally  placed  pulmonary  trunk  (Fig.  259) .  Traced  proximally  they  pursue 
a  spiral  course,  a  shifting  from  left  to  ventrad,  and  c  from  right  to  dorsad,  and 
becoming  continuous  with  the  proximal  swellings.  Thickenings  b  and  d  are  also 
prominent  at  one  point  proximally  and  when  the  bulb  in  this  region  is  divided  by 
ingrowing  connective  tissue  into  the  aorta  and  pulmonary  artery,  the  aorta  con- 
tains the  whole  of  thickenings  &  and  half  of  a  and  c,  while  the  pulmonary  trunk 
contains  the  whole  of  d  and  half  of  a  and  c  (Fig.  260).  The  three  thickenings 
now  present  in  each  vessel  hollow  out  on  their  distal  surfaces  and  eventually  form 


Jorta. 


j4orhz 


Pul 


artery 


Fig.  260. — Scheme  showing  division  of  bulbus  cordis  and  its  thickenings  into  aorta  and  pulmonary 
artery  with  their  valves.  The  division  begins  in  B,  the  lateral  thickenings  dividing  respectively  into 
a,  e,  and  c,f.     Rotation  from  right  to  left  shown  in  D  (Heisler). 


the  thin-walled  semilunar  valves  (Fig.  260).  The  anlages  of  these  valves  are 
prominent  in  embryos  of  10  to  15  mm.  as  thick  plump  swellings  projecting  into 
the  lumina  of  the  aorta  and  pulmonary  artery. 

The  two  proximal  bulbar  swellings  fuse  and  continue  the  spiral  division  of  the 
bulb  toward  the  interventricular  septum  in  such  a  way  that  the  base  of  the  pul- 
monary trunk,  now  ventrad  and  to  the  right,  opens  into  the  right  ventricle,  while 
the  base  of  the  aorta,  now  lying  to  the  left  and  dorsad,  opens  into  the  left  ven- 
tricle close  to  the  interventricular  foramen  through  which  the  two  ventricles 
still  communicate  (Fig.  259,  B). 

Closure  of  the  Interventricular  Foramen. — The  interventricular  foramen  in 
embryos  of  15  to  16  mm.  is  bounded  (1)  by  the  interventricular  septum;  (2)  by 
the  proximal  septum  of  the  bulb;  and  (3)  by  the  dorsal  portion  of  the  fused  endo- 
cardial cushions.     Soon  these  structures  are  approximated,  fuse  and  by  the  de- 


PRIMITIVE    BLOOD   VASCULAR    SYSTEM 


267 


velopment  of  the  septum  membranaceum  the  interventri<  ular  foramen  is  closed. 
The  atrio-vcntricular  valves  arise  as  thickenings  of  the  endocardium  and  endo- 
cardial cushions  of  the  atrio-ventricular  foramina.  Three  such  thickenings  are 
formed  on  the  right,  two  on  the  left.  The  anlages  of  the  valves  are  at  first  thick 
and  project  into  the  ventricles.  Later,  as  the  ventricular  wall  differentiates,  the 
valvular  anlages  are  undermined,  leaving  their  edges  attached  to  the  ventricular 
walls  by  muscular  trabecular  or  cords.  The  muscle  tissue  of  both  the  valves  and 
trabecule  soon  degenerates  and  is  replaced  by  connective  tissue,  forming  the  chor- 
da- tendineac  of  the  adult  valves.  Thus  there  are  developed  the  three  cusps  of 
the  tricuspid  valve  between  the  right  chambers  of  the  heart,  and  the  two  flaps  of 
the  bicuspid  or  mitral  valve  between  the  left  atrium  and  left  ventricle. 

PRIMITIVE  BLOOD  VASCULAR  SYSTEM 
It  is  assumed  that  the  first  paired  vessels  of  human  embryos  are  formed  as 
longitudinal  anastomoses  of  capillary  networks  which  originate  first  in  the  angio- 


Dorsal  segmental  artines 
Umbilical  veins 


Descending  aorfae 


Umbilical  arteries 


Body-stalk 
Umbilical  vein 


Vitelline  arter 


Primitive 
aortic  arch 
•Primitive  heart 

ViUllo-umbilical  frunK 

Vitelline  veins 

Yolk-sac 


Fig.  261. — Diagram,  lateral  view,  of  the  primitive  blood-vessels  in  embryos  of  1.5  to  2  mm.  (adapted 

from  Felix). 


blast  of  the  yolk-sac  and  chorion.  In  an  embryo  of  1.3  mm.  in  which  the  somites 
were  not  yet  developed  (Fig.  261)  the  paired  vessels  are  already  formed.  They 
are  the  umbilical  veins  which  emerge  from  the  chorion,  fuse  in  the  body-stalk,  then, 
separating,  course  in  the  somatopleure  to  the  paired  tubular  heart.  From  the 
heart  tube  paired  vessels  as  ventral  aortce  extend  cephalad,  then  bend  dorsad  as 
the  first  aortic  arches  and  extend  caudad  as  the  dorsal  or  descending  aorta.  These 
bend  sharply  ventrad  into  the  belly  stalk  and  branch  in  the  wall  of  the  chorion. 
The  chorionic  circulation  is  thus  the  first  to  be  established. 

In  embryos  2  to  2.5  mm.  long  (5  to  8  somites)  the  heart  has  become  a  single 
tube  (Fig.  262).     From  the  yolk-sac  numerous  veins  converge  cephalad  and  form 


268 


THE   DEVELOPMENT   OF    THE   VASCULAR   SYSTEM 


a  pair  of  vitelline  veins.  These  join  the  umbilical  veins  and,  as  the  vitello-umbili- 
cal  trunk,  traverse  the  septum  transversum  and  open  into  the  sinus  venosus. 
The  descending  aortae  give  off  dorsally  and  cranially  several  pairs  of  dorsal 
intersegmental  arteries  and  ventrad  and  caudad  a  series  of  non-segmental  vitelline 
arteries  to  the  yolk-sac.     The  umbilical  arteries  now  take  their  origin  from  a  plexus 


Dorsal  intersegmental  arteries 
Umbilical  arteries 


Ant.  cardinal  veins 

Descending  cwrtcu 


Body-stalk 

Umbilical  vein 


ortic  arch  I 


\Heart 
Wifello-  umbilical  frunK 
Witel/ine  veins 
i  YolK-sac 

Fig.  262. — Diagram,  lateral  view,  of  the  primitive  blood-vessels  in  embryos  of  2  to  2.5  mm.  (adapted 

from  Felix). 


Vitelline  arteries 


Posterior  cardinal  veins 
Vitelline  artery 


Ant.  cardinal  veins 

descending  aorta 


Umbilical  arteries 


Bcdy-stolh  I  /      \       \  ^Ueart 

Umbilical  veins     ^^___^/  \         \ 

Vitelline  Veins   \  ^Sinus  Venosus 

Fig.  263. — Diagram  of  the  blood-vessels  of  embryos  with  15  to  23  somites  (modified  from  Felix). 


of  ventral  vessels  in  series  with  the  vitelline  arteries.     At  this  stage  the  vitelline 
circulation  of  the  yolk-sac  is  established. 

In  embryos  of  15  to  23  somites  (Fig.  263)  the  veins  of  the  embryo  proper  de- 
velop as  longitudinal  anastomoses  of  branches  from  the  segmental  arteries.  The 
paired  anterior  cardinal  veins  of  the  head  are  developed  first,  and  coursing  back 


PRIMITIVE   BLOOD   VASCULAR    SYSTEM 


269 


on  cither  side  of  the  brain  they  join  the  vitello-umbilical  trunk.  In  embryos  of 
23  somites  the  posterior  cardinals  are  present.  They  lie  dorsal  to  the  nephrotomy 
and,  running  eraniallv,  join  the  anterior  cardinal  veins  to  form  the  common  cardinal 
veins.  ( taring  to  the  later  enlargement  of  the  sinus  venosus,  the  proximal  portions 
of  the  venous  trunks  are  taken  up  into  its  wall  and  thus  three  veins  open  into  each 
horn  of  the  sinus  venosus:    (1)  the  umbilical  veins  from  the  chorion;    (2)  the 


/*'  Cervical  artery 
Pulmonary  artery 


Dorsal 
aorta 


Vertebral  ar/iry 

0 to  cyst 


Vena  copilii 
media 


Caudal  artery 
Umbilical  artery 


Inf.  mesenteric  artery 

Fig.  264. — Arteries  and  cardinal  veins  of  the  right  side  in  a  4.9  mm.  human  embryo  (modified  after 
Ingalls).     II,  heart;  I,  77,  III,  IV,  and  VI,  first,  second,  third,  fourth,  and  sixth  aortic  arches. 


vitelline  veins  from  the  yolk-sac;  (3)  the  common  cardinal  veins  from  the  body  of 
the  embryo. 

The  descending  aortce  have  now  fused  caudal  to  the  seventh  intersegmental 
arteries  and  form  the  single  dorsal  aorta  as  far  caudad  as  the  origins  of  the  um- 
bilical arteries. 

Of  the  numerous  vitelline  arteries,  one  pair  are  prominent  and  they  fuse  to 
form  the  single  vessel  which  courses  in  the  mesentery  and  later  forms  the  superioi- 


270 


THE   DEVELOPMENT   OP    THE   VASCULAR   SYSTEM 


mesenteric  artery.     By  the  enlargement  of  capillaries  connecting  the  ventral  and 
dorsal  aortae  a  second  pair  of  aortic  arches  is  formed  at  this  stage  (Fig.  263). 

Development  and  Transformation  of  the  Aortic  Arches. — In  embryos  4  to 
5  mm.  in  length  five  pairs  of  aortic  arches  are  successively  developed,  the  first, 
second,  third,  fourth  and  sixth  (Fig.  264) .  An  additional  pair  of  transitory  vessels 
which  extend  from  the  ventral  aorta  to  the  sixth  arch  appear  later  in  embryos  of 


Aortic  arch  3 


Aortic  arch 
2. 

Aortic  arch 
1 


Dorsal  aorta 
Aortic  arch  4- 
Aortic  arch  6 

Esophagus 
Trachea 


ary  artery 


Aortic  arch   3 . 


Int.  carotid  artery 
Aortic  archZ 


Aortic  arch  6 
Pulmonary  artery 

Bui  bus  cordis 


Aortic  arch  4 
Aortic  arch 5 

Dorsal  aorta- 


Fig.  265. — Aortic  arches  of  human  embryos:  A ,  of  5  mm.;  B,  of  7  mm.  (after  Tandler).    I.  II,  III,  IV, 

pharyngeal   pouches. 

7  mm.  but  soon  degenerate  (Fig.  265.  B).  They  are  interpreted  as  being  the  fifth 
pair  in  the  series.  From  each  dorsal  or  descending  aorta  there  develop  cranially 
the  internal  carotid  arteries.  These  extend  toward  the  optic  stalks  where  they 
bend  dorsad  and  caudad,  connecting  finally  with  the  first  intersegmental  arteries 
of  each  side.  The  descending  aortae  are  now  fused  to  their  extreme  caudal  ends 
and  the  umbilical  arteries  take  their  origin  ventrally.  Twenty-seven  pairs  of 
dorsal  intersegmental  arteries  are  present.     From  the  seventh  cervical  pair  of  these 


PRIMITIVE   BLOOD   VASCULAR   SYSTEM 


271 


arteries  arise  the  subclavian  arteries  of  the  upper  limbs.  Of  the  ventral  vitelline 
vessels  three  are  now  prominent,  the  celiac  artery  in  the  stomach-pancreas  re- 
gion, the  vitelline  or  superior  mesenteric  in  the  small  intestine  region  and  the  in- 
ferior mesenteric  of  the  large  intestine  region. 

Of  the  aortic  arches  the  third  pair  is  largest  at  5  mm.  From  the  sixth  pair 
are  given  off  the  small  pulmonary  arteries  to  the  lungs.  At  7  mm.  the  first  and 
second  aortic  arches  are  obliterated  (Figs.  265,  B,  and  266),  but  the  dorsal  and  ven- 
tral aorta?  cranial  to  the  third  arch  persist  as  parts  of  the  internal  and  external 
arteries  respectively.  The  third  arches  form  the  stems  of  the  internal  carotids, 
while  the  ventral  aortae  between  the  third  and  fourth  arches  become  the  common 


Externa/  Carotid 


Ventral  aorta. 


Eight  subclavian 
artery  ~— 


Right 

pulmonary 

artery 

Trunk  of 


pulmonary 
1  artery 


Internal  carotid 
Common  carotid 
Aortic   arch 

Ductus  arteriosus 
Vertebral  artery 

/Subclavian  artery 
Left  pulmonary 
artery 

Dorsal  aorta. 


Fig.  266. — Diagram  showing  the  aortic  arches  and  their  derivatives  in  human  embryos. 


carotids.  In  embryos  of  15  mm.  the  bulbus  cordis  has  been  divided  into  the  aor- 
tic and  pulmonary  trunks  so  that  the  aorta  opens  into  the  left  ventricle  and  the 
pulmonary  trunk  into  the  right  ventricle.  The  dorsal  aortae  between  the  third 
and  fourth  arches  disappear,  but  the  fourth  arch  on  the  left  side  persists  as  the 
aortic  arch  of  the  adult.  On  the  right  side,  the  fourth  aortic  arch  persists  with 
the  descending  aorta  as  far  as  the  seventh  intersegmental  artery  and  forms  part  of 
the  right  subclavian  artery,  which  is  thus  longer  than  the  left.  On  the  right  side, 
the  sixth  arch  between  the  origin  of  the  right  pulmonary  artery  and  descending 
aorta  is  early  lost ;  on  the  left  side,  it  persists  as  the  ductus  arteriosus  and  its  lumen 
is  only  obliterated  after  birth.     The  proximal  portion  of  the  right  sixth  arch 


272 


THE    DEVELOPMENT    OF    THE    VASCULAR    SYSTEM 


forms  the  stem  of  the  right  pulmonary  artery,  but  the  proximal  portion  of  the 
left  arch  is  incorporated  in  the  pulmonary  trunk. 

The  aortic  arches  of  the  embryo  are  of  especial  importance  comparatively,  as  five  arches 
are  formed  in  connection  with  the  gills  of  adult  fishes,  three  are  represented  on  either  side  in 
adult  amphibia  and  reptiles,  while  in  birds  the  right,  in  mammals  the  left,  fourth  arch  persists 
as  the  arch  of  the  aorta. 

From  the  primitive  aortae  arise  dorsal,  lateral  and  ventral  branches  (Fig.  267). 
The  dorsal  branches  are  intersegmental  and  develop  small  dorsal  and  large  ventral 
rami.  From  the  dorsal  rami  are  given  off  neural  branches  which  bifurcate 
and  form  dorsal  and  ventral  spinal  arteries.  Those  of  each  side  anastomose 
longitudinally  in  the  median  line  and  give  rise  to  the  dorsal  and  ventral  median 


Post  transverse 
anastomosis 

Dorsal  ramus 
of  dorsal 

interSeomenU 
a.1  artery/ 

Pre coital 

anastomosis 

[Ventre - 
llatera.1 
)  Visceral 
I  artery 

fLat  ramus 
I  of  .Ventral 
\aiv  -dors at 
mtersegmenta 
L  artery 

D.  splanchnic 
anastomosis 


Preneura.1  anastomosis 

Tost-  costal  anastomosis 
~~    of  dorsal  ramus  of  dorsaJ. 
't'nterseatTteri1a.L  artery 

Dorsal  intersegmental 
artery 

Dorsal  aorta. 

Ventral  splanchnic 
artery 


ventral  anastomosis  of 

ventral  div  of  dorsal  intersegmental  artery 

Fig.  267. — A  diagram  showing  the  arteries  of  the  trunk  in  transverse  section. 


spinal  arteries.     The  dorsal  rami  also  form  lateral  anastomoses  dorsal  and  ven- 
tral to  the  transverse  processes  of  the  vertebrae. 

Origin  of  the  Vertebral  Arteries  and  Basilar  Artery. — As  we  have  seen  (Fig. 
264),  the  internal  carotids  are  recurved  cranially  in  the  5  mm.  embryo  and  anas- 
tomose with  the  first  two  pairs  of  dorsal  intersegmental  arteries.  The  ventral 
longitudinal  anastomosis  of  the  dorsal  rami  of  the  first  seven  pairs  of  dorsal  in- 
tersegmental arteries  gives  rise  to  the  vertebral  arteries  (Fig.  268,  ^4).  The  trunks 
of  the  first  six  pairs  are  lost  so  that  the  vertebrals  take  their  origin  with  the  sub- 
clavians  from  the  seventh  pair  of  intersegmental  arteries  (Fig.  268,  B).  In  embryos 
of  9  mm.  the  vertebrals  in  the  region  of  the  metencephalon  fuse  to  form  a  single 
median  ventral  vessel,  the  basilar  artery,  which  thus  is  connected  cranially  with 
the  internal  carotids,  caudad  with  the  vertebral  arteries. 


PRIMITIVE    BLOOD    VASCULAR    SYSTEM 


273 


slvc.b. 


V?,  ISp.G. 


-  .1  .V.C.V. 


Fig.  268. — A ,  The  development  of  the  vertebral  artery  in  a  rabbit  embryo  of  twelve  days  (Hochstetter 
from  McMurrich).  ///  AB  to  VI  AB,  branchial  arch  vessels;  Ap,  pulmonary  artery;  A.v.c.b.  and 
A.vx.v.,  cephalic  and  cervical  portions  of  vertebral  artery;  A.s.,  subclavian  artery;  C.d.  and  C.i.,  in- 
ternal and  external  carotid  arteries.  I.Sp.G.,  spinal  ganglion.  B,  Arterial  system  of  an  embryo  of 
10  mm.  (His).  Ic,  internal  carotid;  P,  pulmonary  artery;  Ve,  vertebral  artery;  ///  to  VI,  persistent 
aortic  arches. 

18 


274 


THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 


The  internal  carotids,  after  giving  off  the  ophthalmic  arteries,  give  rise  cranially  to  the 
anterior  cerebral  artery,  from  which  arise  later  the  middle  cerebral  artery  and  the  anterior  chorioidal 
artery,  all  of  which  supply  the  brain.  Caudalward  many  small  branches  to  the  brain  wall 
are  given  off  and  quite  late  in  development  (48  mm.  embryos)  these  form  a  true  posterior 
cerebral  artery  (Mall). 

The  ventral  branches  of  the  dorsal  intersegmental  arteries  become  large  in  the 
thoracic  and  lumbar  regions,  and  persist  as  the  intercostal  and  lumbar  arteries, 
segmentally  arranged  in  the  adult.  The  subclavian  and  a  portion  of  the  internal 
mammary  artery  are  derived  from  the  ventral  ramus  of  the  seventh  cervical  seg- 
mental artery.  The  remainder  of  the  in- 
ternal mammary  and  the  superior  and  in- 
ferior epigastric  arteries  are  formed  by  longi- 
tudinal anastomoses  between  the  extremities 
of  the  ventral  rami  from  the  thoracic  and 
lumbar  intersegmental  arteries,  beginning 
with  the  second  or  third  thoracic  (Fig.  269). 
The  superior  intercostal  arteries  arise  from 
anastomoses  which  connect  the  lateral  rami 
of  these  same  branches  on  what  will  later  be 
the  inner  surfaces  of  the  dorsal  portions  of 
the  ribs. 

The  lateral  branches  of  the  descending 
aortae  are  not  segmentally  arranged.  They  sup- 
ply structures  arising  from  the  nephrotome 
region  (mesonephros,  sexual  glands,  meta- 
nephros  and  suprarenal  glands) .  From  them 
later  arise  the  renal,  suprarenal,  inferior  phre- 
nic and  internal  spermatic  or  ovarian  arteries. 
The  ventral  arteries  are  not  definitely  segmental  or  intersegmental.  Primi- 
tively they  form  the  paired  vitelline  arteries  to  the  yolk-sac  (Figs.  261  and  263). 
Coincident  with  the  degeneration  of  the  yolk-sac  the  prolongations  of  the  ventral 
vessels  to  its  walls  disappear  and  the  paired  persisting  arteries,  passing  in  the 
mesentery  to  the  gut,  fuse  to  form  unpaired  vessels  from  which  three  large 
arteries  are  derived,  the  cceliac  artery,  the  superior  mesenteric  and  the  inferior 
mesenteric  (Fig.  264). 

The  primitive  cceliac  axis  arises  opposite  the  seventh  intersegmental  artery.  Together 
with  the  mesenteric  arteries,  it  migrates  caudalward  until  eventually  its  origin  is  opposite 
the  twelfth  thoracic  segment  (Mall).    This  migration,  according  to  Evans,  is  due  to  the  unequal 


Fig.  269. — The  development  of  the 
internal  mammary  and  deep  epigastric  ar- 
teries in  an  embryo  of  13  mm.  (Mall  from 
McMurrich's  Human  Body). 


PRIMITIVE    BLOOD   VASCULAR    SYSTEM 


275 


growth  of  the  dorsal  and  ventral  walls  of  the  aorta.     The  mesenteric  arteries  are  displaced 
caudad  only  three  segments,  probably  in  the  same  way. 

The  Umbilical  and  Iliac  Arteries. — As  previously  described,  the  umbilical 
arteries  arise  in  young  human  embryos  of  2  to  2.5  mm.  from  the  primitive  aorke 
opposite  the  fourth  cervical  segment.  They  take  origin  from  a  plexus  of  ventral 
vessels  of  the  vitelline  series  (Fig.  263),  and  are  gradually  shifted  caudalward  until 
they  arise  from  the  dorsal  aorta  opposite  the  twenty-third  segment  (fourth  lum- 
bar). In  5  mm.  embryos  the  umbilical  arteries  develop  secondary  lateral  con- 
nections with  the  aorta  (Fig.  270,  .4).     The  new  vessels  pass  lateral  to  the  mes- 


7&1  Seamen  taJ  artery 
Coeliac  artery 
V.  pancreas 

VolK-stalK 
Vitelline  artery 

Mesonephnc  arteries. 
Dorsal  aorta 

R.  umbilical  artery 
Cloaca, 


Common  iliac  artery 


lo"1   Dorsal  segmental  arterij 
,  Dorsal  aorta 

,Cot\iae  axis 


'ilellint  artery 


A  B 

Fig.  270. — Reconstructions  showing  the  development  of  the  umbilical  and  iliac  arteries: 
embryo;    B,  9  mm.  embryo  (after  Tandler). 


A,  5  mm. 


onephric  ducts,  and  in  9  mm.  embryos  the  primitive  ventral  umbilical  artery  has 
disappeared.  From  the  newly  formed  vessel  an  artery  arises  which  becomes  the 
external  iliac  artery  of  the  adult.  The  new  lateral  umbilical  trunk  from  the  aorta 
to  the  origin  of  the  external  iliac  now  becomes  the  common  iliac  artery,  and  shifts 
its  position  to  the  ventral  side  of  the  aorta.  The  remainder  of  the  umbilical 
trunk  constitutes  the  hypogastric  artery. 

Arteries  of  the  Extremities. — It  is  assumed  that  in  man,  as  in  mammals,  the 
first  vessels  of  the  limb  buds  form  a  capillary  plexus. 

Upper  Extremity. — The  capillary  plexus  takes  its  origin  by  several  lateral  branches  from 
the  aorta.  In  human  embryos  of  5  mm.  but  one  connecting  vessel  remains  and  this  takes  its 
origin  secondarily  from  the  seventh  intersegmental  artery,  forming  the  ventral  branch  of  this 
artery  and  its  lateral  ramus.    The  portion  of  this  vessel  in  what  will  become  the  free  arm  is 


276 


THE   DEVELOPMENT   OF    THE   VASCULAR   SYSTEM 


plexiform  at  first,  and  later  becomes  a  single  stem  which  forms  successively  the  subclavian, 
axillary,  brachial,  and  interosseous  arteries.  Later,  in  the  arm  are  formed  the  median,  radial 
and  ulnar  arteries.  For  details  as  to  their  arrangement  students  are  referred  to  textbooks  of 
anatomy. 

Arteries  of  the  Lower  Extremity. — In  embryos  of  7  mm.  there  is  given  off  from  the  secondary 
lateral  trunk  of  the  umbilical  artery  a  small  branch  which  forms  the  chief  stem  of  the  vascular 
plexus  in  the  lower  extremity.  This,  the  arteria  ischiadica,  is  superseded  in  embryos  of  15.5 
mm.  by  the  external  iliac  and  femoral  arteries.  The  arteria  ischiadica  persists  as  the  inferior 
gluteal.  We  have  already  seen  that  the  secondary  lateral  root  of  the  umbilical  artery  becomes 
the  common  iliac. 

DEVELOPMENT  OF  THE  VEINS 
We  have  seen  that  in  embryos  of  23  somites  three  systems  of  paired  veins 
are  present,  the  umbilical  veins  from  the  chorion,  the  vitelline  veins  from  the  yolk- 
sac,  and  the  cardinal  veins,  anterior  and  posterior,  which  unite  in  the  common 


Atrium 

Com.  cardinal  vein 

Sinusoids  of  liver 

R.  vitelline  vein 


Ventricle 

L.  umbilical  vein 
L.  vitelline  vein 


Fig.  271. — Reconstruction  of  the  veins  and  arterial  arches  of  a  4.2  mm.  embryo  in  ventral  view  (His). 


cardinal  veins,  from  the  body  of  the  embryo.     Thus  three  veins  open  into  the 
right  and  three  into  the  left  horn  of  the  sinus  venosus  (Fig.  263). 

Charges  in  the  Vitelline  and  Umbilical  Veins. — Vena  porta. — With  the  in- 
crease in  size  of  the  liver  anlages  there  is  an  intercrescence  of  the  hepatic  cords 


DEVELOPMENT   OF   THE   VEINS 


277 


and  the  endothelium  of  the  vitelline  veins.  As  a  result,  these  veins  form  in  the 
liver  a  network  of  sinusoids  (Fig.  271),  and  each  is  divided  into  a  distal  portion 
which  passes  from  the  yolk-sac  to  the  liver  and  into  a  proximal  portion  which  carries 
blood  from  the  liver  sinusoids  to  the  sinus  venosus.  The  proximal  portion  of  the 
left  vitelline  vein  soon  is  largely  absorbed  into  the  sinusoids  of  the  liver  and  shifts 
its  blood  flow  into  the  right  horn  of  the  sinus  venosus.  In  the  meantime  the 
liver  tissue  grows  laterally,  comes  into  contact  with  the  umbilical  veins  and  taps 
them  so  that  their  blood  flows  more  directly  to  the  heart  through  the  sinusoids  of 
the  liver  (Fig.  272).  As  the  channel  of  the  right  proximal  vitelline  is  larger,  the 
blood  from  the  left  umbilical  vein  flows  diagonally  to  the  right  horn  of  the  sinus 


Ductus  venosus 


Right  horn  sinus  venosus 


Right  umbilical 

Distal  venous 
Right 


Left  horn  sinus  venosus 
Left  vitelline  vein 


Proximal  venous  ring 


Middle  venous  ring 


mbilical  vein 


it  vitelline  vein 


Fig.  272. — Reconstruction  of  the  veins  of  the  liver  in  a  4.9  mm.  human  embryo  (after  Ingalls). 


venosus.  When  all  the  umbilical  blood  enters  the  liver,  as  in  embryos  of  5  to  6 
mm.,  the  proximal  portions  of  the  umbilical  veins  atrophy  and  disappear  (Fig. 
273).  In  5  mm.  embryos  the  vitelline  veins  have  formed  three  cross  anastomoses 
with  each  other:  (1)  a  cranial  transverse  connection  in  the  liver  ventral  to  the 
duodenum;  (2)  a  middle  one  dorsal  to  the  duodenum;  and  (3)  a  caudal  one  ven- 
tral to  it.  There  are  thus  formed  about  the  gut  a  cranial  and  a  caudal  venous 
loop  (Fig.  273).  In  embryos  of  7  mm.  the  vitelline  and  umbilical  blood  flows 
chiefly  to  the  left  side.  As  a  result,  the  left  umbilical  and  vitelline  veins  have 
enlarged,  while  the  corresponding  right  veins  have  degenerated.  Of  the  right 
vitelline  vein  only  the  right  limb  of  the  cranial  loop  persists  caudal  to  the  liver. 


278 


THE   DEVELOPMENT   OF    THE   VASCULAR   SYSTEM 


The  left  vitelline  vein  is  present  except  for  the  left  limb  of  the  cranial  loop.  A 
new  vein,  the  superior  mesenteric,  develops  in  the  mesentery  of  the  intestinal  loop 
and  joins  the  left  vitelline  vein  near  the  point  of  its  dorsal  middle  connection  with 
the  right  vitelline  vein.  Subsequently,  with  the  atrophy  of  the  yolk-sac  the  left 
vitelline  vein  degenerates  caudal  to  its  junction  with  the  superior  mesenteric  vein. 
The  persisting  trunk  from  the  superior  mesenteric  vein  to  the  liver  is  the  vena 

porta,  and  thus  represents  (1)  a 
yH  VE      PA     VH  va"  portion  of  the  left  vitelline  vein 

in  the  left  limb  of  the  caudal 
loop;  (2)  the  middle  transverse 
anastomosis  between  the  vitel- 
line veins;  (3)  the  portion  of 
the  right  vitelline  vein  which 
forms  the  right  limb  of  the 
cranial  loop. 

In  the  liver  the  portal  vein 
through  its  cranial  and  ventral 
anastomosis  between  the  vitel- 
line veins  is  connected  with  the 
left  umbilical  vein.  As  the 
right  lobe  of  the  liver  grows, 
the  course  of  the  umbilical  and 
portal  blood  through  the  intra- 
hepatic portion  of  the  right 
vitelline  vein  becomes  circui- 
tous, and  a  new  direct  channel 
to  the  sinus  venosus  is  formed 
through  the  hepatic  sinusoids. 
This  is  the  ductus  venosus  Arantii,  which  is  obliterated  after  birth  and  forms  the 
ligamentum  venosum  of  the  post-natal  liver. 


Fig.  273. — A  diagram  showing  the  development  of  the 
portal  vein  (His  in  Marshall's  Embryology).  PA,  pan- 
creas; 77,  intestine;  TS,  stomach;  VA,  left  umbilical 
vein;  VA',  right  umbilical  vein;  VA",  cranial  detached 
portions  of  umbilical  veins;  VE,  ductus  venosus;  VE, 
efferent  hepatic  vein  derived  from  right  vitelline;  VL, 
afferent  hepatic  vein;  VO,  trunk  of  portal  vein  derived 
from  left  vitelline;  VV,  right  vitelline  vein;  VV,  on  right 
side  of  figure,  superior  mesenteric  vein;  VV,  VV",  por- 
tions of  right  and  left  vitelline  veins  which  atrophy;  W, 
liver;  WD,  bile  duct. 


According  to  Mall,  the  intra-hepatic  portion  of  the  right  vitelline  vein  persists  proximally 
as  the  right  ramus  of  the  hepatic  vein  and  distally  as  the  ramus  arcuatus  of  the  portal  vein. 
The  intra-hepatic  portion  of  the  left  vitelline  vein  drains  secondarily  into  the  right  horn  of  the 
sinus  venosus  and  proximally  forms  later  the  left  hepatic  ramus.  Distally,  where  it  is  connected 
with  the  left  umbilical  vein,  it  becomes  the  ramus  angularis  of  the  vena  porta.  In  this  way  two 
primitive  portal  or  supplying  trunks  and  two  hepatic  or  draining  trunks  originate.  Later  are 
differentiated  first  four,  then  six,  such  trunks  within  the  liver  and  the  six  primary  lobules 
supplied  and  drained  by  these  trunks  may  be  recognized  in  the  adult  liver. 


DEVELOPMENT    OF    THE    VEINS  279 

Of  the  umbilical  veins  the  right  early  disappears;  the  left  persists  during 
fetal  life,  shifts  to  the  median  line  and  courses  in  the  free  edge  of  the  falciform 
ligament.  After  birth  its  lumen  is  closed  and  from  the  umbilicus  to  the  liver  it 
forms  the  ligamentum  teres.  In  early  stages  veins  from  the  body  wall  drain  into 
the  umbilical  veins. 

The  Anterior  Cardinal  Veins  and  the  Origin  of  the  Vena  Cava  Superior. — 
The  anterior  cardinal  veins  consist  each  of  two  parts:  (i)  The  true  anterior  cardi- 
nals located  laterad  in  the  segmented  portion  of  the  head  and  neck  and  draining 
into  the  common  cardinal  veins;  (2)  the  vena  capitis  medialis  extending  into  the 
unsegmented  head  proper  and  running  ventro-lateral  to  the  brain  wall.  In  em- 
bryos of  20  mm.  there  has  formed  by  anastomosis  a  large  connection  between  the 
right  and  left  anterior  cardinals,  which  carries  the  blood  from  the  left  side  of  the 
head  into  the  right  vein  (Fig.  274,  C).  Soon  the  left  anterior  cardinal  loses  its 
connection  with  the  common  cardinal  on  the  left  side  (Fig.  274,  D).  The  proxi- 
mal portion  of  the  left  common  cardinal  with  the  transverse  portion  of  the  sinus 
venosus  persists  as  the  coronary  sinus.  The  right  common  cardinal  and  the  right 
anterior  cardinal  vein  as  far  as  its  anastomosis  with  the  left  anterior  cardinal  be- 
come the  superior  vena  cava.  The  anastomosis  itself  forms  the  left  vena  anonyma, 
while  that  portion  of  the  right  anterior  cardinal  between  the  left  vena  anonyma 
and  the  right  subclavian  vein  is  known  as  the  right  vena  anonyma.  The  distal 
portions  of  the  anterior  cardinals  become  the  internal  jugular  veins  of  the  adult, 
while  the  external  jugulars  are  new  veins  which  develop  much  later. 

The  vena  capitis  medialis  is  the  continuation  of  the  anterior  cardinal  vein  into  the  head  of 
the  embryo  where  at  first  it  lies  mesial  to  the  cerebral  nerves.  Later  it  is  partly  shifted  by 
anastomosis  lateral  to  the  cerebral  nerves  and  forms  the  vena  capitis  lateralis  (Figs.  275,  276). 
In  11  mm.  embryos  this  emerges  with  the  n.  facialis  and  caudal  to  the  n.  hypoglossus  becomes 
the  internal  jugular.  Cranially  the  median  vein  of  the  head  persists  as  the  sinus  cavernosas 
and  receives  the  ophthalmic  vein  from  the  eye,  and  the  middle  cerebral  vein  from  the  fore-  and 
mid-brain  regions.  Between  the  n.  trigeminus  and  the  facialis,  the  middle  cerebral  vein  from  the 
metencephalon  (cerebellum)  joins  the  v.  capitis  lateralis  before  it  leaves  the  cranium.  More 
caudally  the  posterior  cerebral  vein  from  the  myelencephalon  emerges  through  the  jugular  for- 
amen and  i:-.  drained  with  the  others  by  the  v.  capitis  lateralis  into  the  internal  jugular  (Fig. 
276,  B).  Soon  the  three  cerebral  veins  reach  the  dorsal  median  line  (Fig.  276,0,  and  longitudinal 
anastomoses  are  formed:  (1)  between  the  anterior  and  middle  cerebral  veins,  giving  rise  to  the 
superior  sagittal  sinus;  and  (2)  between  the  middle  and  posterior  cerebral  veins  forming  the  greater 
part  of  the  lateral  sinuses.  In  embryos  of  33  mm.  the  v.  capitis  lateralis  disappears  and  the 
blood  from  the  brain  passes  through  the  superior  sagittal  and  lateral  sinuses  and  is  drained  by 
way  of  the  jugular  foramen  into  the  internal  jugular  vein  (Fig.  276,  C,  D).  The  middle  cerebral 
vein  becomes  the  superior  petrosal  sinus,  but  the  inferior  petrosal  sinus  is  formed  as  a  new  channel 
median  to  the  internal  ear.  For  a  more  detailed  account  of  the  origin  of  the  cephalic  veins 
the  student  is  referred  to  the  original  work  of  Mall  (Amer.  Jour.  Anat.,  vol.  4,  1905). 


280 


THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 


The  Posterior  Cardinal  Veins  and  the  Origin  of  the  Inferior  Vena  Cava. — 

The  posterior  cardinal  veins  course  cephalad  along  the  dorsal  side  of  the  mesone- 
phroi  and  open  into  the  common  cardinal  veins  (Fig.  274,  A).     Each  receives  an 


R.Arit.  cardinal vein 

R.Post. Cardinal  vein. 
R  Com.  cardinal  Vein. 


Subclavian  Vein 

Rv  1  tell  me  vein 

R.  Umbilj^al 
vein 

R.Fbst.  Cardinal 
vein 


Nltsonephros 

R.iubcardinal 
Vein 


R.fom.  iliac  Vein 

Caudal  vein 


RAri.cardinal  vein 


R.Subcardina.1 
vein 


ff.com  cardinal 

Vein 


Inf.  vena  cava. 

R.  Post  Cardinal 
Vein 

Mesonephros 

ff.Subcardinal 
Vein 
Metanephro. 


R.  Supra  cardinal 
Vein 


R.  ischiadic  vein 

R.vaudai  vein 


Ed  jugular  Vein 
Subclavian  vein 


Int.  jugular  vein 
Com.  Cardinal  vein 


J)  Ext.jugular  Vein. 

Subclavian  vein 


Azygos  Vein 


Voronaru  sinus 

Inf.vena  cava 

Hepalic  Vein 
Hemi-azyyos  vein 

R -Suprarenal  vein 

R.  renal  vein 

L. renal  Vein 

Spermatic     veins 

Ext  iliac  vein 
Com.  iliac   vein 


Int. jugular  vein 

L.vena  anon y  ma. 


jdupra  Carat 
Vein 

Mttancph 

Post        , 
Cardinal 

vein 
Ischiadic  vein 

Caudal  vein 

Fig.  274. — Four  diagrams  showing  the  development  of  the  superior  and  inferior  venae  cava?  and  the 
fate  of  the  cardinal  veins  (modified  after  Kollmann).  X  in  A,  anastomosis  between  hepatic  and  sub- 
cardinal  vein;  *,  anastomosis  between  subcardinal  veins;  X  in  C,  anastomosis  between  anterior  car- 
dinal veins  which  forms  the  left  vena  anonyma;  *  in  C,  cranial  anastomosis  between  the  posterior  car- 
dinal veins;  2,  caudal  anastomosis  between  the  same  veins;  K,  kidney;  S,  suprarenal  gland;   T,  testis. 


Int.  iliac  veins 

AJedian  Saenz/  i/e/n 


DEVELOPMENT    OF    THE    VEINS 


28l 


internal  iliac  vein  from  the  posterior  extremities,  mesonepJirie  branches  from  the 
mid-kidney  and  dorsal  segmental  veins  from  the  body  wall  (Fig.  274,  B).     Median 


Fig.  275. — Veins  of  the  head,  A,  in  a  9  mm.  human  embryo;  B,  in  an  1 1  mm.  embryo    (Mall). 


and  ventral  to  the  mesonephros  are  developed  the  subcardinal  veins  which  are 
connected  at  intervals  with  the  posterior  cardinal  veins  by  sinusoids  and  with 
each  other  by  anastomoses  ventral  to  the  aorta.     Thus  all  the  blood  from  the 


282 


THE   DEVELOPMENT    OF    THE    VASCULAR    SYSTEM 


mesonephroi,  posterior  extremities  and  dorsal  body  wall  is  in  early  stages  drained 
by  the  posterior  cardinal  veins  alone. 

The  development  of  the  unpaired  vena  cava  inferior  begins  when  communica- 
tion is  established  between  the  right  hepatic  vein  of  the  liver  and  the  right  sub- 
cardinal  vein  of  the  mesonephros,  primarily  a  tributary  of  the  posterior  cardinal 
vein  (Lewis,  1902). 

The  liver  on  the  right  side  becomes  attached  to  the  dorsal  body  wall  and  from 
its  point  of  union  a  ridge,  the  plica  venae  cavae  (Fig.  192),  extends  caudalward. 
According  to  Davis  (1910),  capillaries  from  the  subcardinal  vein  invade  the  plica 


Vcnfluente  of  the  sinuses 


Auditory  vesicle 


ddfe  cerebral  Vein 


Vena  capitis  mediali's 
Trigeminal 


B 


Anterior  cerebral,  vein 
Confluence    ofthe 

minuses 
Superior 
Jayittal 


Middle  cerebral  vein 


cerebral 
vein 


Inferior  Sagittal  sinus 
Great  cerebral  vein 


capitis  . 
a   u-t-     lateralis 
Auditory 
jemmal     Vesicle 
'nerve 

Straight  Sinus 


Trigeminal  nerve 

Ophthalmic   Vein 


D 


petrosal 


Ophthalir. 


Auditory  vesicle 
Inf.  petrosal  sinus 
Trigeminal  nerve 


Fig.  276. — Four  diagrams  showing  the  development  of  the  veins  of  the  head  (after  Mall).     A,  at  four 
weeks;  B,  at  five  weeks;  C,  at  the  beginning  of  the  third  month;  D,  from  an  older  fetus. 


venae  cavae  and,  growing  cranially,  meet  and  fuse  with    capillaries   extending 
caudad  from  the  liver  sinusoids. 

Thus  is  formed  the  vein  of  the  plica  vence  cavce,  which  is  already  present  in 
human  embryos  of  2.6  mm.  (Kollmann).  This  vein  rapidly  enlarges  as  also  do 
the  sinusoidal  connections  between  the  subcardinals  and  posterior  cardinals  at 
one  point.  Thus  the  blood  from  the  lower  posterior  cardinals  is  soon  carried  to 
the  heart,  chiefly  by  way  of  the  right  subcardinal  and  right  hepatic  veins  (Fig. 
B,  274  ).  Soon  the  posterior  cardinals  just  cranial  to  their  enlarged  anastomoses 
with  the  subcardinals  become  small  and  are  interrupted.     Cranial  to  their  inter- 


DEVEI.OPMKNT    OF    TIIK    VEINS  283 

ruption  these  veins  were  formerly  believed  to  persist  as  the  vv.  azygos  and  hemi- 
azygos of  the  adult  (Fig.  274,  C). 

Sabin  (Anal.  Record,  vol.  8,  p.  82,  1014)  has  confirmed  the  conclusions  of  Parker  and 
Tozier  (Bull.  Museum  Comp.  Zool.,  Harvard,  1908)  that  the  vv.  azygos  and  hemiazygos 
arc  new  veins.  They  arc  formed  in  pig  embryos  as  longitudinal  anastomoses  ventral  to  the 
vertebral  and  median  to  the  posterior  cardinal  veins  and  open  into  the  upper  ends  of  the  latter. 
Except  for  this  short  upper  region  the  posterior  cardinals  in  the  thoracic  region  drain  into  the 
new  veins  and  become  tributary  to  them.  The  right  upper  posterior  cardinal  vein  drains  into 
the  v.  azygos,  the  left  upper  posterior  cardinal  vein  into  the  v.  hemiazygos. 

The  portions  of  the  posterior  cardinal  veins  caudal  to  their  interruption  re- 
main for  a  while  symmetrical  and  connected  by  anastomoses  (Fig.  274,  C).  Soon 
the  caudal  anastomosis  between  them  enlarges  until  the  blood  from  both  sides  is 
drained  into  the  right  posterior  cardinal  vein  (Fig.  274,  D).  A  branch  of  the  post- 
cardinal  vein  encircles  the  ureter  of  the  metanephros.  This  vein  is  known  as  the 
supracardinal.  Caudal  to  their  transverse  connection  the  right  posterior  cardinal 
becomes  the  right  common  iliac  vein.  The  corresponding  portion  of  the  left  pos- 
terior cardinal  with  the  transverse  anastomosis  becomes  the  longer  left  common 
iliac  vein.  The  blood  from  these  veins  is  now  drained  by  the  unpaired  vena  cava 
inferior  which  is  composed  of  the  following  veins:  (1)  the  common  hepatic  and 
right  hepatic  veins  (primitive  right  vitelline);  (2)  the  vein  of  the  plica  venae 
cava?;  (3)  a  portion  of  the  right  subcardinal  vein;  (4)  the  supracardinal  vein  of 
the  right  side;   (5)  a  portion  of  the  lower  right  posterior  cardinal  vein. 

The  permanent  kidneys  take  up  their  positions  opposite  the  great  anastomosis  between 
the  posterior  cardinals  and  the  subcardinals  and  at  this  point  the  renal  veins  are  developed. 
On  the  left  side,  the  anastomosis  connecting  the  right  subcardinal  with  the  left  posterior  cardinal 
persists  as  part  of  the  left  renal  vein.  A  persisting  portion  of  the  lower  left  posterior  cardinal, 
according  to  Hochstetter,  forms  the  proximal  part  of  the  left  spermatic  or  ovarian  veins.  The 
dorsal  segmental  veins  of  the  lower  posterior  cardinals  form  the  lumbar  veins.  Transverse 
anastomoses  connect  those  of  the  left  side  with  the  right  posterior  cardinal  after  the  atrophy 
of  the  left  posterior  cardinal.  The  cephalic  portion  of  the  left  subcardinal  vein  persists  as  the 
suprarenal  vein,  which  thus  opens  into  the  renal  vein  instead  of  joining  the  vena  cava  inferior 
as  does  the  right  suprarenal  vein. 

The  Veins  of  the  Extremities. — The  primitive  capillary  plexus  of  the  upper  and  lower 
limb  buds  gives  rise  to  a  border  vein  (Fig.  277),  which  courses  about  the  periphery  of  the  flat- 
tened limb  buds  (Hochstetter).  In  the  upper  extremity,  the  ulnar  portion  of  the  border  vein 
persists,  forming  at  different  points  the  subclavian,  axillary,  brachial  and  basilic  veins.  The 
border  vein  at  first  opens  into  the  dorsal  wall  of  the  posterior  cardinal  vein  (embryos  of  10  mm.), 
but,  as  the  heart  shifts  its  position  caudalward  it  finally  drains  by  a  ventral  connection  into 
the  anterior  cardinal  or  internal  jugular  vein  (F.  T.  Lewis).  The  cephalic  vein  develops  second- 
arily in  connection  with  the  ulnar  border  vein,  later  in  embryos  of  23  mm.  anastomoses  with  the 
external  jugular  and  finally  drains  into  the  axillary  vein  as  in  the  adult.    With  the  development 


284 


THE    DEVELOPMENT    OF    THE    VASCULAR    SYSTEM 


of  the  digits,  the  w.  ccphalica  et  basilica  become  distinct  as  in  embryos  of  35  mm.  but  later  are 
a°-ain  connected  by  a  plexus  on  the  dorsum  mam,  as  in  the  adult  (Evans  in  Keibel  and  Mall). 
In  the  lower  extremity  the  fibular  border  vein  persists  as  the  v.  saphena  parva  which  runs 
deep  as  the  v.  glutca  inferior  and  drains  into  the  hypogastric  portion  of  the  posterior  vena  cava. 
The  v.  saphena  magna  and  the  v.  femoral  is  arise  later  and  join  the  v.  ischiadica  which  drains  into 
the  posterior  cardinal  vein.    The  veins  to  accompany  the  arteries  are  the  last  to  develop. 


Dorsal  subclavian 
vein 


Vena  ulnaris 
prima. 


V.lmquo- facial '/s 


V.  I'm  quo-  facialis 

Ant.  cardinal  vein 

Dorsal  .subclavian 

vein 
Com.  cardinal 
l/ein 


V.thoraco  -  epi  (jastrica. 


V.uL 


nans  prim 


Post. cardinal  vein 

V.  thoraco-  eplqastrica 

Ventral  subclavian  vein 


Vceph, 


I wque- facialis 


■qulan's  interna. 


Vjuqularh  externa. 
V.Cephalica 


Fig.  277. — Four  reconstructions  of  the  veins  of  the  right  arm  (after  F.  T.  Lewis).     A,  10  mm.  embryo; 
B,  11. s  mm.  embryo;    C,  15  mm.  embryo;    D,  22.8  mm.  embryo. 


FETAL  CIRCULATION 

During  fetal  life  the  placental  blood  enters  the  embryo  by  way  of  the  large 

umbilical  vein  and  is  conveyed  to  the  liver  (Fig.  278).     There  it  mingles  with  the 

small  amount  of  venous  blood  brought  to  the  liver  by  the  portal  vein.     It  is  carried 

to  the  inferior  vena  cava  either  directly,  through  the  ductus  venosus,  or  indirectly 


FETAL  CIRCULATION 


285 


through   the   liver   sinusoids   and    hepatic  vein,   and  is  again  mixed  with   the 
venous  blood.     Entering  the  right  atrium   it  mingles  more  or  less  with  the 
venous  blood  which  enters  the  atrium  through  the  superior  vena  cava.     From 
the    right   atrium    the    blood    may   take 
two  paths.     That  from  the  inferior  vena 
cava  is  said  to  be  directed  by  the  valve 
of   this  vein  -through  the  foramen   ovale 
into  the  left  atrium,  which,  before  birth, 
receives    little    venous    blood   from    the 
lungs.      The  venous    blood  of  the  supe- 
rior vena  cava,  somewhat   mixed,  is  sup- 
posed to  pass  from  the  right  atrium  into 
the  right  ventricle. 

The  purer  blood  of  the  left  atrium 
enters  the  left  ventricle,  whence  it  is 
driven  out  through  the  aorta  and  dis- 
tributed chiefly  to  the  head  and  upper 
extremities.  The  mixed  blood  of  the  right 
ventricle  passes  out  by  the  pulmonary 
artery.  A  small  amount  of  this  blood  is 
conveyed  to  the  lungs  by  the  pulmonary 
arteries,  but,  as  the  fetal  lungs  do  not 
function,  most  of  it  passes  to  the  dorsal 
aorta  by  way  of  the  ductus  arteriosus  and 
is  distributed  to  the  trunk,  viscera  and 
lower  extremities.     The  placental  circuit 

is  completed   by  the   hypogastric  or  um- 

r  '  ' r    °  Fig.  278. — Diagrammatic  outline  of  the 

organs  of  circulation  in  the  fetus  of  six  months 
(Allen  Thomson).1  RA .  right  atrium  of  the 
heart;  RV,  right  ventricle;  LA,  left  atrium; 
Ev,  valve  of  inf.  vena  cava ;  L V,  left  ventricle;  L,  liver;  K,  left  kidney;  /,  portion  of  small  intestine;  a, 
arch  of  the  aorta;  a',  its  dorsal  part;  a",  lower  end;  ves,  superior  vena  cava;  vci,  inferior  vena  where  it 
joins  the  right  atrium;  vci',  its  lower  end;  s,  subclavian  vessels;  j,  right  jugular  vein;  c,  common  ca- 
rotid arteries;  four  curved  dotted  arrow  lines  are  carried  through  the  aortic  and  pulmonary  opening  and 
the  atrioventricular  orifices;  da,  opposite  to  the  one  passing  through  the  pulmonary  artery,  marks  the 
place  of  the  ductus  arteriosus;  a  similar  arrow  line  is  shown  passing  from  the  vena  cava  inferior 
through  the  fossa  ovalis  of  the  right  atrium  and  the  foramen  ovale  into  the  left  atrium;  hv,  the  he- 
patic veins;  vp,  vena  porta?;  x  to  vci,  the  ductus  venosus;  uv,  umbilical  vein;  ua,  umbilical  arteries; 
lie,  umbilical  cord  cut  short;   i,  V,  iliac  vessels. 

1  In  this  diagram  the  arteries  are  conventionally  colored  red  and  the  veins  blue,  but  these  colors  are 
not  intended  to  indicate  the  nature  of  the  blood  conveyed  by  the  respective  vessels. 


286  THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 

bilical  arteries,  which  pass  from  the  common  iliac  arteries  by  way  of  the 
umbihcal  cord  to  the  placenta. 

Changes  at  Birth. — At  birth  the  umbilical  vessels  are  ruptured  and  the  lungs 
become  functional.  The  umbilical  arteries  and  veins,  no  longer  used,  contract 
and  their  lumina  are  obliterated  by  the  thickening  of  the  inner  coat  (tunica  in- 
tima) .  The  lumen  of  the  umbilical  arteries  is  occluded  after  four  days,  that  of 
the  umbilical  vein  within  a  week.  The  wall  of  the  vein  is  persistent  as  the  liga- 
mentum  teres  of  the  liver. 

The  ductus  venosus  atrophies  because  after  birth  only  the  blood  from  the  por- 
tal vein  enters  the  liver,  and  this  is  all  drained  into  the  liver  sinusoids,  forming  the 
portal  circulation.  The  ductus  venosus  is  persistent  as  the  fibrous  ligamentum  ve- 
nosum,  embedded  in  the  wall  of  the  liver. 

The  ductus  arteriosus  ceases  to  function  after  birth,  as  all  the  blood  from  the 
pulmonary  arterial  trunk  is  conveyed  to  the  expanded  lungs.  The  ductus  be- 
comes impervious  from  ten  to  twenty  days  after  birth  and  persists  as  a  solid 
fibrous  cord. 

The.  foramen  ovale  does  not  function  after  birth,  as  the  large  amount  of  blood 
returned  to  the  left  atrium  from  the  now  functional  lungs  equalizes  the  pressure 
in  the  two  atria.  As  a  result,  both  during  diastole  and  systole,  the  septum  pri- 
mum,  or  valve  of  the  foramen  ovale,  is  pressed  against  the  septum  secundum,  clos- 
ing the  foramen  ovale.  Eventually  the  two  septa  fuse,  though  they  may  be 
incompletely  united  during  the  first  year  after  birth,  or  even  longer. 


The  Lymphatic  System 

The  development  of  the  lymphatics  is,  according  to  Sabin  (Keibel  and  Mall, 
vol.  2,  p.  709),  divided  into  two  stages:  (1)  the  development  from  the  veins  of 
isolated  lymph-sacs,  which  become  united  with  each  other  by  the  thoracic  duct 
and  acquire  a  secondary  opening  into  the  veins  at  the  jugular  valves;  (2)  the  peri- 
pheral outgrowth  of  lymphatic  vessels  as  endothelial  sprouts  from  the  lymph-sacs. 

In  10  to  11  mm.  embryos  appear  the  jugular  sacs  lateral  to  the  internal  jugu- 
lar veins  and  derived  from  them,  first  as  a  plexus  of  capillaries  which  becomes  iso- 
lated, but  later  rejoins  the  vein,  forming  a  valve  at  the  opening.  In  23  mm.  em- 
bryos the  retro-peritoneal  sac  (F.  T.  Lewis,  190 1-2)  appears  at  the  root  of  the  mes- 
entery adjacent  to  the  suprarenal  bodies  and  caudal  to  the  superior  mesenteric 
artery.  It  is  developed  from  a  capillary  plexus  arising  from  the  neighboring 
veins.     Posterior  lymph-sacs  are  developed  from  the  v.  ischiadica. 


THE    LYMPHATIC    SYSTEM 


287 


In  embryos  of  30  mm.  the  thoracic  duel  has  developed,  connecting  the  left 
jugular  sac  with  the  retroperitoneal  sac  and  receptaculum  chyli  (Fig.  279).  Ac- 
cording to  Lewis,  the  thoracic 
duct  arises  from  the  union  of 
several  detached  lymphatics  de- 
rived originally  from  the  veins. 
The  receptaculum  chyli  "is  a 
secondary  enlargement  dorsal  to 
the  aorta."  In  later  stages  the 
lymph-sacs  are  replaced  by  plex- 
uses of  lymphatic  vessels  which 
grow  to  the  head,  neck,  and  arm 
from  the  jugular  sacs;  to  the  hip, 
back  and  leg  from  the  posterior 
sacs,  and  to  the  mesentery  from 
the  retroperitoneal  sac.  Valves 
are  developed  in  the  lymphatic 
vessels  from  folds  of  their  endo- 
thelium. 


Huntington  and  McClure  be- 
lieve that  the  isolated  lymphatic 
spaces  are  derived  independently  from 
the  mesenchyma.  Lewis  (in  Amer. 
Jour.  Anal.,  1905,  vol.  5,  pp.  95-120) 
rejects  this  view  for  the  following 
reasons: 

"1.  The  lymphatic  spaces  do 
not  resemble  mesenchyma. 

"2.  After  being  formed  the 
lymphatics  increase  like  blood-vessels 
by  means  of  blind  endothelial  sprouts 
and  not  by  connecting  with  intercel- 
lular spaces. 

"3.  In  early  embryos  detached 
blood-vessels  may  be  seen  without 
proving  that  blood-vessels  arc  mesen- 


Fig.  279. — Flat  reconstruction  of  the  primitive  lym- 
phatic system  in  a  human  embryo  30  mm.  long,  Mall 
collection,  No.  86,  X  about  5.4.  Cc,  cistema  chyli;  Lg, 
lymphoglandula;  N.III,  X.  I]',  and  X.  V,  Xn.  cervi- 
cales;  SJ.jug.,  saccus  lymphaticus  posterior;  S.Ls.,  sac- 
cus  lymphaticus  subclavius;  ]'.c,  vena  cephalica;  V.c.i., 
vena  cava  inferior;  V.f.,  vena  femoralis;  V.j.i.,  vena 
jugularis  inferior;  V.l.p.,  vasa  lymphatica  profunda; 
V.l.s..  vasa  lymphatica  superficialis;  Y.r.,  vena  renalis; 
v.s.,  vena  sciatica;  V.u.  (p.),  vena  ulnaris  (primitiva) 
(Sabin). 

chymal  spaces. 

"4.     The  endothelium  of  the  lymphatics  is  sometimes  seen  to  be  continuous  with  that 

of  the  veins." 

The  Lymph  Glands. — These  are  developed  first  as  plexuses  of  lymphatics 
in  connection  with  an  artery  and  vein.     The  first  pair  are  formed  in  the  axillary 


288 


THE  DEVELOPMENT  OF  THE  VASCULAR  SYSTEM 


region  from  the  jugular  sacs.  Connective-tissue  septa  occur  between  the  vessels 
of  the  lymphatic  network.  Next  lymphocytes  collect  in  the  connective  tissue  and 
a  capillary  network  is  formed  connected  with  an  artery  and  vein.  The  lymphocytes 
multiply  and  form  lymphoid  tissue  (Fig.  280,  A),  and  a  peripheral  lymph  sinus  is 
developed  with  afferent  and  efferent  lymphatics.  The  blood-vessels  enter  the 
lymph  gland  at  one  point,  the  hilus.  Soon  the  lymphatic  vessels  invade  the 
lymphoid  tissue  and  form  anastomosing  channels,  so  that  the  lymph  enters  at  the 
periphery  and  is  drained  through  the  hilus.  In  the  larger  glands  (Fig.  280,  B) 
the  connective  tissue  forms  a  definite  capsule  and  extends  through  the  gland  as 
cords  or  trabeculce,  in  which  course  the  larger  blood-vessels.  At  the  periphery  of 
the  gland  the  lymphocytes  divide  actively  and  form  dense  lymph  nodules  with 


.•■.-•■■•'■  -  ■.'.©•'•'<§;»■•:■•.■ 

T  ~3?  ®&$*£r&i '"'  Lig-  hepatogastric 


Crnlom  epitheV~ \»<fe) 


•  •  Ccelom  epithel. 


t&Gk"  Mesoderm 


Pancreas 


— JiWi ;&£«w8£» ■<&■■  Mesog.  post. 

w 

^Tuy— -  Cozlom  epithet. 


Fig.   281. — Two  stages  in  the  early  development  of  the  spleen:    A,  from  an 
(Kollmann);    B,  from  a  20  mm.  embryo  (Tonkoff). 


10.5  mm. 


germinal  centers  wherein  the  lymphocytes  are  actively  dividing.  The  peripheral 
nodules  constitute  the  cortex  of  the  gland.  At  the  center  of  the  gland,  and  near 
the  hilus,  the  network  of  lymph  sinuses  divides  the  lymphoid  tissue  into  anasto- 
mosing cords  and  this  region  of  the  gland  becomes  the  medulla. 

Haemolymph  glands,  according  to  Schumacher,  begin  their  development 
like  lymph  glands,  but  soon  after  the  formation  of  the  peripheral  sinus  the  lym- 
phatic connections  degenerate  and  the  blood  escapes  from  the  blood  capillaries 
into  the  sinuses. 

The  Spleen. — Little  is  known  of  the  early  development  of  the  spleen  in  hu- 
man embryos  beyond  the  fact  that  it  originates  as  a  thickening  of  the  dorsal 
mesogastrium  due  to  the  division  of  the  cells  of  the  peritoneal  epithelium  (Fig. 
281).     The  spleen  is  a  lymphoid  organ  in  which  blood  sinuses,  instead  of  lymph 


Afferent  lymphatu   vessels 


<< 


Network  of  lymph- 
atic vessels 


Young  connective 

tissue 


Lymphoid  tissui 


r 


Lymphatic  vessel       Blood  vessels       Lymphatic  vessel 


Peripheral  lymph 

sinus 
Capsule 

Trabecule 

Reticular  tissue 
cells 


Lymph  si)ius 


B 

Afferent  lymphatic  vessels 


Peripheral  sinus 


Capsule  ' 


SltfArt-yy 


Secondary  nodule 


Medullary  cord 


Trabecule 


Efferent  lymphatic  vessels 

Fig.  2S0—  Diagrams  representing  four  stages  in  the  development  of  lymph  glands.     The  earlier  stages 
are  shown  on  the  left  side  of  each  figure  (Lewis  and  Stohr) 


THE    I.VMl  IIA  IK     SYSTEM  289 

sinuses,  are  developed  to  form  the  spaces  of  the  splenic  pulp.  Mall  has  shown 
by  injecting  pig  embryos  that  in  the  younger  fetuses  the  blood-vessels  of  the  spleen 

form  a  closed  system.  In  fetuses  of  10  to  12  cm.  the  capillaries  enlarge,  giving 
rise  to  definite  capillary  units  which  drain  into  the  veins  through  openings  in 
their  syncytial  endothelium.  The  enlarged  cavernous  capillaries  form  the  spaces 
in  the  splenic  pulp. 

Lifschitz  has  shown  that,  in  human  embryos  between  15  and  30  cm.  long,  red 
blood-cells  are  actively  formed  in  the  splenic  pulp  around  the  giant  cells.  The 
lymphoid  tissue  of  the  spleen  first  appears  as  ellipsoids  about  the  smallest  arteries 
in  fetuses  of  four  months.  At  seven  months  the  ovoid  splenic  corpuscles  appear 
as  lymphoid  nodules  about  the  larger  arteries.  The  lymphoid  tissue  is  not 
formed  from  tissue  of  the  blood-vessels,  but,  like  the  lymph  nodules  of  lymph 
glands,  is  developed  around  them  from  the  mesenchyma. 


19 


CHAPTER  X 

HISTOGENESIS 

The  primitive  cells  of  the  embryo  are  alike  in  structure.  The  protoplasm  of 
each  exhibits  the  fundamental  properties  of  irritability,  contractility,  conductivity 
and  metabolism  (the  absorption,  digestion,  and  assimilation  of  nutritive  sub- 
stances and  the  excretion  of  waste  products,  processes  through  which  growth 
and  reproduction  are  made  possible).  As  development  proceeds,  there  is  a 
gradual  differentiation  of  the  cells  into  tissues,  each  tissue  being  composed  of  like 
cells,  the  structure  of  which  has  been  adapted  to  the  performance  of  a  certain 
special  function.  In  other  words,  there  is  division  of  labor  and  adaptation  of  cell 
structure  to  the  function  which  each  cell  performs.  The  differentiation  of  tissue 
cells  from  the  primitive  cells  of  the  embryo  is  known  as  histogenesis.  On  page  64 
the  derivatives  of  the  germ  layers  are  given.  We  shall  take  up  briefly  the  histo- 
genesis of  the  tissues  derived  from  the  endoderm,  mesoderm,  and  ectoderm  in  the 
order  named. 

The  Histogenesis  of  the  Entodermal  Epithelium 

The  cells  of  the  entoderm  are  little  modified  from  their  primitive  structure. 
From  the  first  they  are  concerned  with  the  processes  of  absorption,  digestion, 
assimilation  and  excretion.  They  form  always  epithelial  layers  lining  the  di- 
gestive and  respiratory  canals  and  the  glandular  derivatives  of  these.  In  the 
pharynx,  esophagus  and  trachea  the  cells  are  early  of  columnar  form  and  ciliated. 
The  epithelium  of  the  pharynx  and  esophagus  becomes  stratified  and  the  surface 
layers  flatten  to  form  squamous  cells.  The  stratified  epithelium  is  developed 
from  a  basal  germinal  layer  like  the  epidermis  of  the  integument  (see  p.  304). 
Throughout  the  rest  of  the  digestive  canal  the  simple  columnar  epithelium  of  the 
embryo  persists.  At  the  free  ends  of  the  majority  of  the  cells  is  developed  a  cutic- 
ular  plate.  Other  cells  are  converted  into  unicellular  mucous  glands  or  goblet 
cells.  As  outgrowths  of  the  intestinal  epithelium,  are  developed  the  simple  tu- 
bular glands  of  the  stomach  and  intestine  and  the  liver  and  pancreas. 

In  the  respiratory  tract  the  entoderm  forms  at  first  a  simple  columnar  epi- 
thelium. Later,  in  the  trachea  and  bronchi  this  is  differentiated  into  a  pseudo- 

290 


HISTOGENESIS    OF    THE   MESODERMAL   TISSUES 


291 


stratified  ciliated  epithelium.  The  columnar  epithelium  of  the  alveoli  and  al- 
veolar ducts  of  the  lungs  is  converted  into  the  flattened  squamous  respiratory 
epithelium.  The  development  of  the  thymus  and  thyreoid  glands,  liver  and 
pancreas  has  been  described  in  Chapter  VII. 


Histogenesis  of  the  Mesodermal  Tissues 

The  differentiation  of  the  mesoderm  has  been  described  on  p.  61,  Fig.  51. 
It  gives  rise  to  the  mesodermal  segments,  intermediate  cell  masses,  somatic  and 
splanchnic  layers,  all  of  which  are  epithelia,  and  to  the  diffuse  mesenchyme.     The 


Mesodermal 
segment 

Central  cells  of 
segment 

Sclerotome 
Ectoderm 

Intermediate 
cell  mass 


Urogenital  ridge 

Splanchnic 
mesoderm 


Somatic 
mesoderm 


Splanchnic 

mesoderm 


Fig.   2S2. — Transverse  section  of   a  4.5  mm.  embryo  showing  the  development  of   the  sclerotomes 

(Kollmann's   Handatlas). 

somatic  and  splanchnic  layers  of  the  mesoderm  form  on  their  ccelomic  surfaces  a 
single  layer  of  squamous  cells  termed  the  mesot helium.  This  is  the  covering  layer 
of  the  pericardium,  pleura,  peritoneum,  mesenteries,  serous  layer  of  the  viscera 
and  lining  of  the  ^aginEi-sae  4n^lT3C4"otmni  From  this  mesothelium  is  derived 
also  the  epithelium  of  the  genital  glands  and  that  of  the  Muellerian  ducts. 

The  intermediate  cell  masses  or  nephrotomes  are  the  anlages  of  the  pro- 
nephros, mesonephros.  metanephros,  and  their  ducts  (p.  203). 

The  Sclerotomes  and  Mesenchyme. — The  cavities  of  the  mesodermal  seg- 
ments become  filled  with  diffuse  spindle-shaped  cells,  then  their  median  walls  are 
converted  into  similar  tissue  which  migrates  mesially  towards,  and  eventually 
surrounds,  the  neural  tube  and  notochord  (Fig.    282).     This  diffuse  tissue   is 


292  HISTOGENESIS 

mesenchyme  (see  p.  63),  and  that  derived  from  a  single  mesodermal  segment  con- 
stitutes a  sclerotome.  The  sclerotomes  ultimately  are  converted  into  connective 
tissue,  into  the  vertebrae,  and  into  the  basal  portion  of  the  cranium.  The  per- 
sisting lateral  plate  of  the  mesodermal  segment  becomes  a  dermo-myotome,  from 
which  the  voluntary  muscle  is  differentiated  and  probably  the  dermis  of  the  in- 
tegument. 

In  the  head  region  cranial  to  the  otocysts  no  mesodermal  segments  are  formed, 
but  the  primitive  mesoderm  is  converted  directly  into  mesenchyme.  Mesen- 
chyme is  derived  also  from  the  somatic  and  splanchnic  mesoderm  and  from  the 
primitive  streak  tissue.  From  the  mesenchyme  a  number  of  tissues  are  developed 
(see  p.  64).  The  origin  of  the  blood  and  primitive  blood-vessels  and  lymphatics 
has  been  described;  it  remains  to  trace  the  development  of  the  supporting  tissues 
(connective  tissue,  cartilage  and  bone)  and  of  the  smooth  muscle  fibers. 


THE  SUPPORTING  TISSUES 

The  supporting  tissues  are  peculiar  in  that  during  their  development  from 
the  mesenchyme  a  fibrous,  hyaline  or  calcified  matrix  is  formed  which  becomes 
greater  in  amount  than  the  persisting  cellular  elements  of  the  tissue. 

Connective  Tissue. — Different  views  are  held  as  to  the  differentiation  of  con- 
nective-tissue fibers.  According  to  Laguess  and  Merkel,  the  fibers  arise  in  an 
intercellular  matrix  derived  from  the  cytoplasm  of  mesenchymal  cells.  Szily 
holds  that  fibers  are  first  formed  as  processes  of  epithelial  cells  and  that  into  this 
fibrous  meshwork  mesenchymal  cells  later  migrate.  The  view  generally  ac- 
cepted, that  of  Flemming,  Mall,  Spalteholz  and  Meves,  is  that  the  primitive  con- 
nective-tissue fibers  are  developed  as  a  part  of  the  cell,  i.  e.,  are  intracellular  in  origin. 

The  mesenchyme  is  at  first  compact,  the  cell  nuclei  predominating.  Soon  a 
syncytium  is  developed,  the  cytoplasm  increasing  in  amount  and  forming  an  open 
network.  Next  the  cytoplasm  is  differentiated  into  a  perinuclear  granular 
endo plasm  and  an  outer  distinct  hyaline  layer  of  ectoplasm  (Fig.  283,  A).  In  the 
ectoplasm  fibrils  appear,  derived  from  coarse  filaments  known  as  chondrioconta 
(Meves) . 

Reticular  Tissue. — Single  fibers  of  reticulin  arise  in  the  ectoplasm  of  the 
mesenchymal  syncytium.  The  nuclei  and  endoplasm  persist  as  reticular  cells. 
According  to  Mall,  reticular  fibers  differ  chemically  from  white  connective-tissue 
fibers 

White  Fibrous  Connective  Tissue.— The  differentiation  of  this  tissue  may  be 


THE    SUPPORTING    TISSUES 


293 


divided  into  two  stages:  (1)  a  prefibrous  stage  during  which  the  ectoplasm  is 
formed  rapidly  by  the  endoplasm  of  the  cells,  and  fibrils  resembling  those  of 
reticular  tissue  appear  in  the  ectoplasm  (Fig.  283,  A).  (2)  The  anastomosing 
fibers  take  the  form  of  parallel  bundles  and  are  converted  through  a  chemical 
change  into  typical  white  fibers.  The  spindle-shaped  cells  are  transformed  into 
the  connective-tissue  cells  characteristic  of  the  adult.  In  tendons,  the  bundles  of 
white  fibers  are  arranged  in  compact  parallel  fascicles,  in  areolar  tissue  they  are 
interwoven  to  form  a  meshwork.     The  cells  of  the  tendons  are  compressed  be- 


Mesenchymal 
cell 

Fibrillae  in 

ecloplasmic 

matrix. 


\ 

Cell  of.      J. 
Jyncytium        I 

Elastic  A 
fiber 


B 


Mesenchymal  Cell 

Cartilage  matrix 


Cartilage   cell 


Fig.  283.- — Figures  showing  the  differentiation  of  the  supporting  tissues  (after  Mall).  A ,  white  fibers 
forming  in  the  dermis  of  a  5  cm.  pig  embryo;  B,  elastic  fibers  forming  in  the  syncytium  of  the  umbilical 
cord  from  a  7  cm.  embryo;   C,  developing  cartilage  from  the  occipital  bone  of  a  20  mm.  pig  embryo. 


tween  the  bundles  of  fibers  and  this  accounts  for  their  peculiar  form  and  arrange- 
ment. In  the  cornea  of  the  eye  the  cells  retain  their  processes.  The  corneal 
tissue  is  thus  embryonic  in  character  and  is  without  elastic  fibers  or  blood-vessels. 
Elastic  Tissue. — With  the  exception  of  the  cornea  and  tendon,  yellow  elastic 
fibers  develop  in  connection  with  all  white  fibrous  connective  tissue.  Like  the 
white  fibers  they  are  produced  in  the  ectoplasm  of  the  mesenchymal  syncytium 
(Fig.  283,  B).  They  are  developed  as  single  fibers,  but  may  coalesce  to  form  the 
fenestrated  membranes  of  the  arteries.     According  to  Ranvier,  elastic  fibers  are 


294 


HISTOGENESIS 


produced  by  the  union  of  ectoplasmic  granules,  but  this  view  is  not  supported  by 

either  Mall  or  Spalteholz. 

Adipose  Tissue. — Certain  of  the  mesenchymal  cells  give  rise  not  to  fibro- 
blasts but  to  fat-cells.  They  secrete  within  their  cyto- 
plasm droplets  of  fat  which  increase  in  size  and  become 
confluent  (Fig.  284).  Finally,  a  single  fat  globule  fills 
the  cell  of  which  the  nucleus  and  cytoplasm  are  pressed 
to  the  periphery.  The  fat-cells  are  most  numerous 
along  the  course  of  the  blood-vessels  in  areolar  connec- 
tive tissue  and  appear  first  during  the  fourth  month. 


Fig.  284. — Developing 
fat-cells,  the  fat  blackened 
with  osmic  acid  (after 
Ranvier).  n,  nucleus;  g, 
fat  globules. 


CARTILAGE 

Cartilage  has  been  described  as  developing  in  two 
ways:    (1)  The  mesenchymal  cells  increase  in  size  and 
form  a  compact  cellular  precartilage.     Later  the  hyaline 
matrix  is  developed  between  the  cells  from  their  cyto- 
plasm (Fig.  285,  A).    The  matrix  may  in  this  case  be  re- 
garded as  the  ectoplasm  of  the  cartilage  cells.     (2)  Ac- 
cording to  Mall,  mesenchymal  cells  give  rise  first  to  an  ectoplasm  in  which 
fibrillae  develop.     Next,  the  cells  increase  in  size  and  are  gradually  extruded  until 
they  lie  in  the  spaces  of  the  ecto- 

Mo»-  Pre.Cart: 


Carl. 


plasmic  matrix  (Figs.  283C,  285,  B). 
Simultaneously,  the  ectoplasm  is 
converted  into  the  hyaline  matrix 
peculiar  to  cartilage,  undergoing 
both  a  chemical  and  structural 
change.  About  the  cartilage  cells 
the  endoplasm  produces  capsules 
of  hyaline  substance. 


The  interstitial  growth  of  cartilage 
is  due:  ( 1)  to  the  production  of  new  hya- 
line matrix;  (2)  to  the  formation  of  cap- 
sules about  the  cells  and  their  transforma- 
tion into  matrix;  (3)  to  the  proliferation  of 
the  cartilage  cells,  which  may  separate  or 
occur  in  clusters  within  a  single  capsule. 

Perichondral  growth  also  takes  place  about  the  periphery  of  the  cartilage  and  is  due  to 
the  activity  of  persisting  mesenchymal  cells,  which,  with  an  outer  sheath  of  connective  tissue, 


Fig.  285. — Diagrams  of  the  development  of  carti- 
lage from  mesenchyma  (Lewis  and  Stohr).  A ,  based 
upon  Studnicka's  studies  of  fish;  B,  upon  Mall's  Study 
of  Mammals.  Mes.,  mesenchyma;  Pre.  carl.,  pre- 
cartilage;  Cart.,  cartilage. 


BONK  295 

constitute  the  perichondrium.    When  cartilage  is  replaced  by  bone,  the  perichondrium  becomes 
the  periosteum. 

In  hyaline  cartilage  the  matrix  remains  hyaline.  In  fibro-cartilage  the  fibrillations  of  the 
primitive  ectoplasm  are  converted  into  while  fibers.  In  elastic  cartilage  yellow  elastic  fibers 
are  formed  in  the  hyaline  matrix,  according  to  -Mall;  before  the  hyaline  matrix  is  differentiated, 
according  to  Spalteholz.  Most  of  the  bones  of  the  skeleton  are  laid  down  first  in  the  form  of 
cartilage.     Later,  this  is  gradually  replaced  by  the  development  of  bone  tissue. 


BONE 

Bone  is  a  tissue  appearing  relatively  late  in  the  embryo.  There  are  de- 
veloped two  types,  the  membrane  bones  of  the  face  and  cranium  and  the  cartilage 
bones  which  replace  the  cartilaginous  skeleton.  Cartilage  bones  are  not  simply 
cartilage  transformed  into  bone  by  the  deposition  of  calcium  salts,  but  represent 
a  new  tissue  which  is  developed  as  the  cartilage  is  destroyed. 

Membrane  Bone. — The  bones  of  the  face,  the  parietals,  frontals  and  parts 
of  the  occipital,  temporals  and  sphenoid  are  not  preformed  as  cartilage;  the  man- 
dible is  developed  around  a  pair  of  cartilages  (of  Meckel).  The  form  of  a  mem- 
brane bone  is  determined  by  the  development  of  a  periosteal  membrane  from 
the  mesenchyma.  The  bone  matrix  is  differentiated  within  the  periosteum  from 
enlarged  cells,  the  osteoblasts  (bone-formers) .  Osteoblasts  appear  in  clusters  and 
from  their  cytoplasm  is  differentiated  a  fibrillated  ectoplasmic  matrix  like  that 
which  precedes  the  formation  of  connective  tissue  and  cartilage  (Fig.  286  A). 
This  fibrillated  matrix,  by  a  chemical  change  apparently,  is  converted  into  a 
homogeneous  bone  matrix,  which  first  takes  the  form  of  spicules.  The  spicules 
coalesce,  form  a  network  of  bony  plates  and  constitute  the  bone  matrix  upon  the 
surfaces  of  which  osteoblasts  are  arranged  in  a  single  layer  like  the  cells  of  an 
epithelium  (Fig.  286  B).  These  cells  may  be  cuboidal,  columnar  or  flatten  out  as 
bone  formation  ceases.  As  the  matrix  of  the  bone  is  laid  down,  osteoblasts 
become  enclosed  and  form  bone  cells.  The  bone  cells  are  lodged  in  spaces  termed 
lacuna;.  These  are  connected  by  microscopic  canals,  the  canaliculi,  in  which 
course  delicate  cell  processes  and  anastomose  with  those  of  neighboring  cells. 

The  plates  of  the  spongy  membrane  bone  are  formed  about  blood-vessels 
as  centers.  As  the  bone  grows  at  the  periphery,  the  bone  matrix  is  resorbed 
centrally.  At  this  time  large  multinucleated  cells  (43  to  91  m  long)  appear  upon 
the  surfaces  of  the  bone  matrix.  These  cells  are  known  as  osteoclasts  (bone- 
destroyers).  There  is,  however,  no  positive  evidence  that  the  osteoclasts  are 
active  in  dissolving  the  bone.  They  may  be  interpreted  also  as  degenerating 
osteoblasts.     The  cavities  in  which  they  are  frequently  lodged  are  known  as 


296 


HISTOGENESIS 


Hows  hip's  lacuna;.  The  bone  lamellae  of  the  central  portion  of  the  membrane 
bone  are  gradually  resorbed  and  this  portion  of  the  bone  is  of  a  spongy  texture. 
Some  time  after  birth,  compact  bone  lamellae  are  laid  down  by  the  inner  osteo- 
blast cells  of  the  periosteum.  In  the  case  of  flat  bones,  compact  inner  and  outer 
plates  or  tables  are  thus  developed  with  spongy  bone  between  them.  The  spaces 
in  the  spongy  bone  are  filled  by  derivatives  of  the  mesenchyme:  reticular  tissue, 
blood-vessels,  fat-cells  and  developing  blood-cells.     These  together  constitute 


Bone   matrix 


Osteoblas 


^1  ®f/4^^5^#^ 


_BoA?e  ce// 


Fibrillar  in  bone  matrix 

Fig.  286. — Two  stages  in  the  development  of  bone:  A,  section  through  the  occipital  cartilage  of  a 
20  mm.  pig  embryo  (Mall);  B,  section  through  the  periosteum  and  bone  lamelhe  of  the  mandible  from  a 
65  mm.  human  embryo.     X  325. 


the  red  bone  marrow.     The  ossification  of  membrane  bones  begins  at  the  middle 
of  the  bone  and  proceeds  in  all  directions  from  this  primary  center. 

Cartilage  Bone. — The  form  of  the  cartilage  bone  is  determined  by  the  pre- 
formed cartilage  and  its  surrounding  membrane,  the  perichondrium  (Fig.  288). 
Bone  tissue  is  developed  as  in  membrane  bones  save  that  the  cartilage  is  first 
destroyed  and  the  new  bone  tissue  develops  (i)  in  and  (2)  about  it.  In  the  first 
case,  the  process  is  known  as  endochondral  bone  formation.  In  the  second  case, 
it  is  known  as  perichondral  or  periosteal  bone  formation. 


. 


Fig.  287. — A  longitudinal  section  of  the  two  distal  phalanges  from  the  finger  of  a  five-months'  hu- 
man fetus  (Sobotta)  (X  15)-  K>i,  Cartilage  showing  calcification  and  resorption;  eK,  endochondral 
bone;  .1/,  marrow  cavity;   pK.  periosteal  bone. 


BONE 


2Q7 


Endochondral  Bone  Formation. — The  cartilage  cells  enlarge,  become  ar- 
ranged in  characteristic  rows  and  resorb  the  cartilage  matrix  (Fig.  287).  The 
perichondrium  becomes  the  periosteum.  From  its  inner  or  osteogenic  layer, 
which  is  densely  cellular,  ingrowths  invade  the  cartilage  as  it  is  resorbed  and  fill 
the  primary  cavities.  The  invading  osteogenic  tissue  gives  rise  to  osteoblasts 
and  bone  marrow.  By  the  osteoblasts  bone  is  differentiated  directly  upon 
persisting  portions  of  the  cartilage.  As  new  bone  is  developed  peripherally,  it  is 
resorbed  centrally  to  form  large  marrow  spaces.  Eventually,  all  of  the  cartilage 
matrix  is  destroyed.     The  fate 

C  D 


Cartildiyt 


Cartilage 


Bone  paraphysn 


one  marrow 


\piphys 


— Paraphys 


of    the    cartilage    cells    is    un- 
known. 

Perichondral  Ossification. 
— Compact  bone  is  developed 
after  birth  by  the  osteogenic 
layer  of  the  periosteum  and  thus 
are  produced  the  periosteal  la- 
mella. In  the  ribs  this  is  said 
to  be  the  only  method  of  ossi- 
fication. The  bone  lamellae  de- 
posited about  a  blood-vessel  are 
concentrically  arranged  and 
form  the  concentric  lamella  of  a 
Haversian  system.  The  Haver- 
sian canal  of  adult  bone  is 
merely  the  space  occupied  by 
a  blood-vessel. 

Growth  of  Cartilage  Bones. 
— In  cartilage  bones  there  is  no 

interstitial  growth  as  in  cartilage.  Most  of  the  cartilage  bones  have  more  than  one 
center  of  ossification  and  growth  is  due  to  the  expansion  of  the  inten-ening  cartil- 
age. Flat  bones  grow  at  the  periphery,  ring-like  bones,  such  as  the  vertebrae,  have 
three  primary  centers  of  ossification,  between  which  the  cartilage  continues  to  grow 
(Fig.  288  A).  In  the  case  of  the  numerous  long  bones  of  the  skeleton,  the  primi- 
tive ossification  center  forms  the  shaft  or  diaphysis  (Fig.  288  C-F).  The  cartilage 
at  either  end  of  the  diaphysis  grows  rapidly  and  thus  the  bone  increases  in  length. 
Eventually,  osteogenic  tissue  invades  these  cartilages  and  new  ossification  centers, 
the  epiphyses,  are  formed,  one  at  either  end.     When  the  growth  of  the  bone  in 


-Epiphysis 


FlG.  288. — Diagrams  to  show  the  method  of  growth 
of  .1,  a  vertebra;  B,  of  sacrum;  C-F,  of  a  long  bone 
(the  tibia). 


298  HISTOGENESIS 

length  is  completed,  the  epiphyses,  by  the  ossification  of  the  intervening  cartilage, 
are  united  to  the  diaphysis. 

The  shaft  of  the  long  bones  grows  in  diameter  by  the  peripheral  deposition 
of  bone  lamellae  and  the  central  resorption  of  the  bone.  In  the  larger  long  bones 
spongy,  or  cancellated  bone  tissue  persists  at  the  ends,  but  in  the  middle  portion 
a  large  medullary,  or  marrow  cavity,  is  developed.  This  is  filled  chiefly  with  fat 
cells  and  constitutes  the  yellow  bone  marrow. 

Regeneration  of  Bone. — If  bone  is  injured  or  fractured,  new  bone  is  developed  by 
osteoblasts  derived  either  from  the  periosteum  or  from  the  bone  marrow.  The  repair  of  a 
fracture  is  usually  preceded  by  the  formation  of  cartilage  which  unites  the  ends  of  the  bones 
and  is  later  changed  to  bone.  In  adults,  the  periosteum  is  especially  important  in  the  regener- 
ation of  bone  tissue. 

The  special  development  of  the  various  bones  of  the  skeleton  is  beyond  the  scope  of  this 
book.  The  student  is  referred  to  the  various  text-books  of  anatomy,  to  Kollmann's  Handatlas 
of  Embryology,  vol.  i,  and  to  Bardeen's  chapter  in  Keibel  and  Mall  (vol.  i,  p.  316  ff). 

Joints. — In  joints  of  the  synarthrosis  type  in  which  little  movement  is  allowed 
the  mesenchyma  between  the  ends  of  the  bones  differentiates  into  connective 
tissue  or  cartilage.     This  persists  in  the  adult. 

In  joints  of  the  diarthrosis  type  the  bones  are  freely  movable.  The  mesen- 
chyma between  the  bones  develops  into  an  open  connective  tissue  in  which  a  cleft 
appears,  the  joint  cavity.  The  cells  lining  this  cavity  flatten  out  and  form  a  more 
or  less  continuous  layer  of  epithelium,  the  synovial  membrane.  From  the  con- 
nective tissue  surrounding  the  joint  cavity  are  developed  the  various  fibrous 
ligaments  typical  0f  the  different  joints.    , 

THE  HISTOGENESIS  OF  MUSCLE 

The  muscular  system  is  composed  of  muscle  fibers  which  form  a  tissue  in 
which  contractility  has  become  the  predominating  function.  The  fibers  are  of 
three  types :  (1)  smooth  muscle  cells  found  principally  in  the  walls  of  the  viscera  and 
blood-vessels;  (2)  striated  cardiac  muscle,  forming  the  myocardium  of  the  heart; 
(3)  striated  voluntary  muscle,  chiefly  attached  to  the  elements  of  the  skeleton  and 
producing  voluntary  movements.  All  three  types  are  derived  from  the  meso- 
derm. The  only  exceptions  are  the  smooth  muscle  of  the  iris,  and  the  smooth 
muscle  of  the  sweat  glands,  which  are  derived  from  the  ectoderm. 

Smooth  Muscle  in  general  may  be  said  to  arise  from  the  mesenchyme,  or 
from  embryonal  connective  tissue.  Its  development  has  been  studied  by  McGill 
(Internat.  Monatschr.  f.  Anat.  u.  Physiol.,  vol.  24,  pp.  209-245,  1907)  in  the 
esophagus  of  pig  embryos.     The  stellate  cells  of  the  mesenchyma  enlarge,  elongate 


THE   HISTOGENESIS    OF   MUSCLE  299 

and  their  cytoplasm  becomes  more  abundant.  The  resulting  spindle-shaped  cells 
(Fig.  289  A)  remain  attached  to  each  other  by  cytoplasmic  bridges  and  develop 
in  the  superficial  layer  of  their  cytoplasm  course  myoglia  fibers  (Fig.  289  B) 
similar  to  the  primitive  fibrilke  of  connective  tissue.  The  myoglia  fibers  may 
extend  from  cell  to  cell,  thus  connecting  them.  These  fibers  are  the  products  of 
coalesced  granules  found  within  the  cytoplasm  of  the  myoblasts.  In  embryos 
of  30  mm.  fine  myofibrillae  are  differentiated  in  the  cytoplasm  of  the  myoblasts 
and  give  it  a  longitudinally  striated  appearance.  The  cytoplasmic  processes  of 
the  muscle  cells,  the  cytoplasmic  bridges,  later  give  rise  to  white  connective 
tissue  fibers  which  envelop  the  muscle  fibers  and  bind  them  together.  Smooth 
muscle  increases  in  amount:  (1)  by  the  formation  of  new  fibers  from  the 
mesenchyme  of  the  embryo;  (2)  by  the  transformation  into  muscle  fibers  of 
interstitial  cells;  (3)  by  the  multiplication  of  their  nuclei  by  mitosis  in  the  more 
advanced  fetal  stages. 

Striated  Cardiac  Muscle. — This  is  developed  from  the  splanchnic  mesoderm 
which  forms  both  the  epicardium  and  the  myocardium.  The  cells  of  the  myo- 
cardium at  first  form  a  syncytium  in  which  myofibrillae  develop  from  chondri- 
oconta  or  cytoplasmic  granules.  The  myofibrillae  are  developed  at  the  periphery 
of  the  syncytial  strands  of  cytoplasm  and  extend  long  distances  in  the  syncytium. 
They  multiply  rapidly  in  number  and  become  differentiated  each  into  alternating 
dark  and  light  bands,  due  to  a  difference  in  density.  The  syncytial  character  of 
cardiac  muscle  persists  in  the  adult  and  the  nuclei  remain  central  in  position.  The 
intercalated  discs  typical  of  adult  cardiac  muscle  appear  relatively  late,  just 
before  birth  in  the  guinea-pig,  according  to  Jordan  and  Steele. 

Striated  Voluntary  Muscle. — All  striated  voluntary  muscle  is  derived  from 
the  mesoderm,  either  from  the  myotomes  of  the  segments  (muscles  of  the  trunk) 
or  from  the  mesenchyma  (muscles  of  the  head).  According  to  Bardeen  (in 
Keibel  and  Mall,  vol.  1),  after  the  formation  of  the  sclerotome  (Fig.  282  A),  which 
gives  rise  to  skeletal  tissue,  the  remaining  portion  of  the  primitive  segment  con- 
stitutes the  myotome.  All  the  cells  of  the  myotome  give  rise  to  myoblasts.  Wil- 
liams (Amer.  Jour.  Anat.,  vol.  88),  working  on  the  mesodermal  segments  of 
the  chick,  finds  that  only  the  dorsal  and  mesial  cells  are  myoblasts.  By  multi- 
plication they  form  a  mesial  myotome,  while  the  lateral  cells  of  the  original 
mesodermal  segment  persist  as  a  dermatome  and  give  rise  only  to  the  connective 
tissue  of  the  dermis  (Fig.  291).  The  dermatome  lies  lateral  to  the  myotome 
and  the  two  together  constitute  the  dcrmo-myotomc,  according  to  Williams  (Fig. 

45).    " 


;oo 


HISTOGENESIS 

A 


m.  m. 


^^^/^Z'  vies. 

vsBJr  S 


.^^^^ 


Fig.  289.— Two  stages  in  the  development  of  smooth  muscle  fibers;  A ,  from  the  esophagus  of  a  13 
mm.  pig  (McGill);  5,  a  longitudinal  section  of  the  esophagus  of  a  27  mm.  pig  (after  McGill  in 
Lewis-Stohr).  b,  b.m.,  basement  membrane;  e,  epi,  epithelium;  f.c,  myoglia  fibrils;  g.s.,  granules  coa- 
lescing; h.s.,  homogeneous  fibers;  vies.,  mesenchyma;  mm.,  m'uscularis  mucosae;  mu.,  muscle  cell;  »., 
nerve  cells;  s.c,  circular  smooth  muscle  cut  across;  s.l.,  longitudinal  smooth  muscle  cut  lengthwise. 


THE   HISTOGENESIS    OF   MUSCLE  3OI 

As  to  the  origin  of  the  striated  voluntary  muscle  fibers,  there  is  also  a  differ- 
ence of  opinion.  It  is  generally  believed  that  the  myoblasts  elongate  and,  by 
the  repeated  mitotic  division  of  their  nuclei,  become  multinucleated.  Godlewski 
holds  that  several  myoblasts  unite  to  form  a  single  muscle  fiber.  The  nuclei  lie 
at  first  centrally,  surrounded  by  the  granular  sarcoplasm  in  which  myofibrils 
differentiate  peripherally.  The  myofibrils  become  striated  like  those  of  cardiac 
muscle.  During  development  the  muscle  libers  increase  enormously  in  size,  the 
nuclei  migrate  to  the  surface  and  the  myofibrillar  are  arranged  in  bundles  or 
muscle  columns  (sarcostyles) .  This  arrangement  of  the  fibrillar  may,  however, 
be  due  to  shrinkage  in  the  preparation  of  the  sections  observed. 

According  to  Baldwin  (Zeitschr.  f.  allg.  Physiol.,  vol.  14,  191 2),  the  nucleus  and  perinu- 
clear sarcoplasm  is  separated  from  the  rest  of  the  muscle  fiber  by  the  sarcolemma.  With  Apathy, 
he  would  therefore  regard  the  myofibrillar  as  a  differentiated  product  of  the  muscle  cells  and  to  be 
homologized  with  connective  tissue  fibers.  The  extrusion  of  the  muscle  cell  from  the  muscle 
fiber  may  be  compared  to  the  extrusion  of  cartilage  cells  from  the  precartilage  matrix,  as  de- 
scribed by  Mall  (see  p.  294). 

During  the  later  stages  in  the  development  of  striated  voluntary  muscle,  there  is,  according 
to  many  observers,  an  active  degeneration  of  the  muscle  fibers. 

While  smooth  muscle  fibers  form  a  syncytium  and  the  enveloping  connective 
tissue  is  developed  directly  from  the  muscle  cells,  in  the  case  of  striated  voluntary 
muscle  each  fiber  is  a  multinucleated  entity  which  is  bound  together  with  others 
by  connective  tissue  of  independent  origin. 

Morphogenesis  of  the  Muscles. — The  development  of  the  individual  muscles 
of  the  human  body  has  been  described  in  detail  by  W.  Lewis  (in  Keibel  and  Mall, 
vol.  1,  p.  473)  and  to  this  work  the  student  is  referred.  We  may  state  briefly 
here  the  origin  of  the  muscles  of  the  trunk,  limbs  and  head. 

The  muscles  of  the  trunk. — The  deep  muscles  are  derived  from  the  myo- 
tomes which  extend  ventrally  and  fuse  with  one  another  (Fig.  290).  This  fusion 
is  well  advanced  superficially  in  embryos  of  9  to  10  mm.  The  deep  portions  of 
the  myotomes  do  not  fuse  but  give  rise  to  the  intervertebral  muscles,  which  thus 
retain  their  primitive  segmental  arrangement.  The  various  long  muscles  of  the 
back  arise  by  longitudinal  and  tangential  splitting. 

The  thoraco-abdominal  muscles  arise  as  ventral  extensions  of  the  thoracic 
myotomes  into  the  somatopleure,  growing  in  along  with  the  ribs. 

The  musculature  of  the  extremities. — It  has  been  generally  believed  that  the 
muscles  of  the  extremities  were  developed  from  buds  of  the  myotomes  which  grew 
into  the  anlages  of  the  limbs.  According  to  Lewis,  "there  are  no  observations 
of  distinct  myotome  buds  extending  into  the  limbs."     A  diffuse  migration  of 


302 


HISTOGENESIS 


cells  from  the  ventral  portion  of  the  myotomes  has  been  recorded  by  various 
observers,  recently  by  Ingalls.  These  cells  soon  lose  their  epithelial  character 
and  blend  with  the  undifferentiated  mesenchyma  of  the  limb  buds  (Fig.  291). 
From  this  diffuse  tissue  then  the  limb  muscles  are  differentiated,  the  proximal 
muscles  being  the  first  to  appear. 


Fig.  290. — Reconstruction  of  a  9  mm.  embryo  to  show  the  myotomes  (Bardeen  and  Lewis). 


The  musculature  of  the  head  is  derived  from  the  pre-otic  mesenchymal 
tissue  which  condenses  to  form  premuscle  masses.  No  myotomes  are  developed 
in  this  region. 

The  pharyngeal  muscles  probably  arise  from  the  mesenchymal  tissue  of  the 

third  branchial  arch. 


THE    HISTOGENESIS   OF   THE    ECTODERMAL   DERTVATTVES 


303 


The  intrinsic  muscles  of  the  larynx  arc  differentiated  from  the  mesenchyme 
at  the  ventral  ends  of  the  third  and  fourth  branchial  arches. 

The  muscles  of  the  tongue  arc  supplied  by  the  n.  hypoglossus  and  therefore 


Spinal  ganglion 


Permatome 


Ventral  root 


Myotome 

Spinal  nerve 

Arm  bud 

Proliferating  cells 
of  myotome 

Mesonephric  duet 

M esonephrie  tubule  and 
glomerulus 

Civlom 

Somatic  mesodt  rm 


Fig.  291. — Transverse  section  of  a  10.3  mm.  monkey  embryo  showing  the  myotome  and  the  mesenchyma 
of  the  arm  bud  (Kollmann's  Handatlas).    A,  aorta;  *,  sclerotome. 

it  has  been  assumed  that  they  are  derived  from  myotomes  of  the  occipital  region. 
According  to  W.  Lewis,  ''there  is  no  evidence  whatever  for  this  statement,  and 
we  are  inclined  to  believe  from  our  studies  that  the  tongue  musculature  is  derived 
from  the  mesoderm  of  the  floor  of  the  mouth." 


The  Histogenesis  of  the  Ectodermal  Derivatives 

Besides  forming  the  enamel  of  the  teeth  and  salivary  glands  (see  p.  161),  the 
ectoderm  gives  rise:  (1)  to  the  epidermis  and  its  derivatives  (subcutaneous 
glands,  nails,  hair,  and  lens  and  cornea  of  the  eye) ;  ( 2)  to  the  nervous  system  and 
sensory  epithelia ;  (3)  to  parts  of  certain  glands  producing  internal  secretions  such 
as  the  pituitary  body,  adrenal  glands,  and  chromaffin  bodies.     We  shall  describe 


3°4 


HISTOGENESIS 


here  the  histogenesis  of  the  epidermis  and  the  development  of  its  derivatives  and 
the  histogenesis  of  the  nervous  tissues,  reserving  for  final  chapters  the  develop- 
ment of  the  nervous  organs  and  the  glands  formed  in  part  from  them. 

THE  EPIDERMIS 
The  single-layered  ectoderm  of  the  early  embryo  by  the  division  of  its  cells 
becomes  differentiated  into  a  two-layered  epidermis  composed  of  an  inner  layer 
of  cuboidal  or  columnar  cells,  the  stratum  germinativum,  and  an  outer  layer  of 
flattened  cells,  the  epitrichium  or  periderm  (Fig.  292  A). 


Epitrichhim 

Stratum 
germinativum 

Con  am 


Ep'itrichi 


Intermed- 
iate I  aye T  ""^T 
Stratum . 

Xa^Kflurrn'e    Mill 

Fig.  292. — Sections  of  the  integument  from  a  65  mm.  embryo.  A,  section  through  the  integument 
of  the  neck  showing  a  two-layered  epidermis  and  the  beginning  of  a  third  intermediate  layer;  B,  section 
from  the  integument  of  the  chin  in  which  three  layers  are  well  developed  in  the  epidermis.     X  440. 


The  stratum  germinativum  is  the  reproducing  layer  of  the  epidermis.  As 
development  proceeds,  its  cells  by  division  gradually  give  rise  to  new  layers  above 
it  until  the  epidermis  becomes  a  many  layered  or  stratified  epithelium.  The 
periderm  is  always  the  outermost  layer  of  the  epidermis.  In  embryos  of  30  to 
88  mm.  the  epidermis  is  typically  three-layered,  the  outer  flattened  layer  forming 
the  periderm,  a  middle  layer  of  polygonal  cells,  the  intermediate  layer  and  the 
inner  columnar  layer  being  the  stratum  germinativum  (Fig.  292  B).  This  con- 
dition may  persist  until  the  end  of  the  fourth  month.  After  the  fourth  month 
the  epidermis  becomes  many  layered.  The  inner  layers  of  cells  now  form  the 
stratum  germinativum  and  are  actively  dividing  cells  united  with  each  other  by 


THE   HAIR 


305 


cytoplasmic  bridges.  The  outer  layers  of  cells  become  cornified,  the  cornilkation 
of  the  cells  proceeding  from  the  stratum  germinativum  toward  the  surface.  Thus, 
next  the  germinal  layer  are  cells  containing  keratohyalin,  which  constitute  the 
stratum  granulosum,  a  single  layer  of  cells.  A  thicker  layer  above  the  stratum 
granulosum  shows  cells  in  which  drops  of  a  substance  called  eleidin  are  formed. 
These  droplets,  which  are  supposed  to  represent  softened  keratohyalin,  give  these 
cells  a  clear  appearance  when  examined  unstained.  Hence  the  layer  is  termed 
the  stratum  lucidum.  In  the  outer  layers  of  the  epidermis  the  thickened  walls 
of  the  cells  become  cornified  and  in  the  cells  themselves  a  fatty  substance  collects. 
These  layers  of  cells  constitute  the  stratum  corneum.  The  cells  of  this  layer  are 
also  greatly  flattened,  especially  at  the  surface. 

When  the  hairs  develop  they  do  not  penetrate  the  outer  periderm  layer  of 
the  epidermis  but,  as  they  grow  out,  lift  it  off.  Hence  this  layer  is  known  also 
as  the  epitrichium  (layer  upon  the  hair).  Pigment  granules  appear  soon  after 
birth  in  the  cells  of  the  stratum  granulosum.  These  granules  are  probably  formed 
in  situ.  Negro  children  are  quite  light  in  color  at  birth  but  within  six  weeks  their 
integument  has  reached  the  normal  degree  of  pigmentation. 

The  dermis  or  corium  of  the  integument  is  developed  from  mesenchyme  or 
from  the  dermatomes  of  the  mesodermal  segments.  For  special  points  concern- 
ing its  development  see  Keibel  and  Mall's  "Human  Embryology,"  vol.  1,  p.  254. 


THE  HAIR 

Hairs  are  derived  from  thickenings  of  the  epidermis  and  begin  to  develop  at 
the  end  of  the  second  month  on  the  eyebrows,  upper  lip  and  chin.  The  hair  of 
the  general  body  integument  appears  at  the  beginning  of  the  fourth  month. 

The  first  evidence  of  a  hair  anlage  is  the  elongation  of  a  cluster  of  epidermal 
cells  in  the  inner  germinal  layer  (Fig.  293  A).  The  bases  of  these  cells  project  into 
the  dermis  and,  above  them,  cells  of  the  epidermis  are  arranged  parallel  to  the 
surface.  The  elongated  cells  continue  to  grow  downward  until  a  cylindrical  hair 
anlage  is  produced  (Fig.  293  B,  C).  This  consists  of  an  outer  wall  formed  of  a 
single  layer  of  columnar  cells,  continuous  with  the  basal  layer  of  the  epidermis. 
This  wall  bounds  a  central  mass  of  irregularly  polygonal  epidermal  cells.  About 
the  hair  anlage  the  mesenchyma  forms  a  sheath,  and  at  its  base  a  condensation  of 
mesenchyme  produces  the  anlage  of  the  hair  papilla,  which  projects  into  the 
enlarged  base  of  the  hair  anlage.  As  development  proceeds,  the  hair  anlage  grows 
deeper  into  the  corium  and  its  base  enlarges  to  form  the  hair  bulb  (Fig.  293  C). 


306 


HISTOGENESIS 


The  hair  is  differentiated  from  the  basal  epidermal  cells  surrounding  the  hair 
papilla.     These  cells  elongate  and  grow  centrally  toward  the  surface  distinct 


Epitrichium 


'dermaV. 
ye  of  hair's 


Anlage  of  hair 
papilla. 


Hair  bulb 
Hair  pap'ilh 


Fig.  293. — Section  through  the  integument  of  the  face  of  a  65  mm.  embryo  showing  three  stages  in  the 

early  development  of  the  hair.     X  330. 


Inner  hair 
Sheath 


from  the  peripheral  cells  which  form  the  outer  sheath  of  the  hair  (Fig.  294).     The 
central  core  of  cells  gives  rise  to  the  inner  hair  sheath  and  to  the  shaft  of  the  hair. 

At  the  sides  of  the  outer  hair 
sheath  two  swellings  appear  on 
the  lower  side  of  the  obliquely 
directed  hair  anlage.  The  more 
superficial  of  these  is  the  anlage 
of  the  sebaceous  gland  (Fig.  294) . 
The  deeper  swelling  is  the  "epi- 
thelial bed, "  a  region  where  the 
cells  by  rapid  division  con- 
tribute to  the  growth  of  the 
hair  follicle. 

Superficial  to  the  bulb,  the 
cells  of  the  hair  shaft  become 
cornified  and  differentiated  into 
outer  cuticle,  middle  cortex  and 
central  medulla.  The  inner 
hair  sheath  extends  from  the 


Outer  hair 
Sheath 


Epidermis 


A  rrector  pili 
muscle  fibers 

Oebaceous  gland. 


Mesenchymal 
sheath 


Epithelial  bed 


Root  of  hair 


Hair  bulb 
Hair  papilla 


Fig.  294. — Longitudinal  section  through  a  developing  hair 
from  a  five  and  one-half  months'  fetus  (Stohr). 


SWEAT   GLANDS — MAMMARY    GLANDS  307 

bulb  to  the  level  of  the  sebaceous  glands,  where  it  disappears.  The  hair  grows 
at  the  base  and  is  pushed  out  through  the  central  cavity  of  the  anlage,  the  «  ells 
of  which  degenerate.  When  the  hair  projects  above  the  surface  of  the  epidermis 
it  carries  with  it,  and  breaks  up,  the  cpitridiial  layer.  The  mesenchymal  tissue 
which  surrounds  the  hair  follicle  in  the  neighborhood  of  the  epithelial  bed  gives 
rise  to  the  smooth  fibers  of  the  corrector  pili  muscles.  Pigment  granules  develop 
in  the  basal  cells  of  the  hair  and  give  it  its  characteristic  color. 

The  first  generation  of  hairs  are  short-lived  and  begin  to  degenerate  before 
birth;  usually  the  hair  of  the  head  is  shed  during  the  first  and  second  years  after 
birth,  and  new  hairs  develop  as  buds  from  the  old  hair  follicles. 

SWEAT  GLANDS 
The  sweat  or  sudoriparous  glands  begin  to  develop  in  the  fourth  month  from 
the  epidermis  of  the  finger-tips,  of  the  palms  of  the  hands  and  soles  of  the  feet: 
They  are  formed  as  solid  downgrowths  from  the  epidermis,  but  differ  from  hair 
anlages  in  having  no  mesenchymal  papillae  at  their  bases.  During  the  sixth 
month  the  tubular  anlages  of  the  gland  begin  to  coil  and  in  the  seventh  month 
their  lumina  appear.  The  inner  layer  of  cells  forms  the  gland  cells  while  the  outer 
cells  become  transformed  into  smooth  muscle  fibers  which  here  arise  from  the 
ectoderm.    In  the  axillary  region  sweat  glands  occur  which  are  large  and  branched. 

MAMMARY  GLANDS 
The  tubular  mammary  glands  peculiar  to  mammals  are  regarded  as  modified 
sweat  glands.  In  early  embryos  an  ectodermal  thickening  extends  ventro- 
laterally  between  the  bases  of  the  limb  buds  on  either  side.  This  linear 
epidermal  thickening  is  the  milk  line.  In  the  future  pectoral  region  of  this  line 
by  the  thickening  and  downgrowth  of  the  epidermis  there  is  formed  the  papilla- 
like anlage  of  the  mammary  gland  (Fig.  295  A).  From  this  epithelial  anlage  buds 
appear  (B)  which  elongate  and  form  solid  cords  15  to  20  in  number,  the  anlages 
of  the  milk  ducts  (Fig.  295  C).  These  branch  in  the  mesenchymal  tissue  of  the 
corium  and  eventually  produce  the  alveolar  end-pieces  of  the  mammary  glands. 
In  the  region  where  the  milk  ducts  open  on  the  surface  the  epidermis  is  evagi- 
nated  to  form  the  nipple.  The  glands  enlarge  at  birth,  at  puberty  and  after  par- 
turition when  they  become  functionally  active. 

The  mammary  glands  are  homologised  with  sweat  glands  because  their  development  is 
similar,  and  because  in  the  lower  mammals  their  structure  is  the  same.     In  many  mammals 


;oS 


HISTOGENESIS 


numerous  pairs  of  mammary  glands  are  developed  along  the  milk  line  (pig,  dog,  etc.);  in  some  a 
pair  of  glands  is  developed  in  the  pectoral  region  (primates,  elephants);  in  others  only  in  the 
inguinal  region  (sheep,  cow,  horse).  In  man  supernumerary  mammary  glands  developed  along 
the  milk  line  are  of  not  infrequent  occurrence. 


Fig.  295. — Sections  representing  three  successive  "stages  of  development  of  the  human  mammary 
gland  (Tourneux):  A,  fetus  of  32.40  mm.  (1.3  in.);  B,  of  10.16  cm.  (4  in.);  C,  of  24.35  cm-  (9-6  in.); 
a,  epidermis;  b,  aggregation  of  epidermal  cells  forming  anlage  of  gland;  c,  galactophorous  ducts;  d, 
groove  limiting  glandular  area;  e,  great  pectoral  muscle;  /,  unstriated  muscular  tissue  of  areola;  g, 
subcutaneous  adipose  tissue. 

THE  NAILS 

The  anlages  of  the  nails  proper  are  derived  from  the  epidermis  and  may  be 
recognized  in  embryos  of  45  mm.  A  nail  anlage  forms  on  the  dorsum  of  each 
digit  extending  from  the  tip  of  the  digit  almost  to  the  articulation  of  the  terminal 
phalanx.  At  the  base  of  the  anlage,  that  is,  proximally,  the  epidermis  is  folded 
inward  to  form  the  proximal  nail  fold  (posterior  nail  fold  of  the  adult) .  This  is 
curved,  convex  proximally,  and  extends  transversely  to  the  dorsum  of  the  digit 
(Fig.  296  A,  C).  The  nail  fold  also  extends  laterally  on  either  side  of  the  nail 
anlage  and  forms  the  lateral  nail  fold  of  the  adult  (A,  B) . 

The  matrix  of  the  nail  is  developed  in  the  proximal  nail  fold  (C) .  In  a  layer 
of  epidermal  cells,  lying  parallel  to  the  dorsum  of  the  digit,  there  are  developed 
keratin  or  horn  fibrils  during  the  fifth  month  of  fetal  life.  These  appear  without 
the  previous  formation  of  keratohyalin  granules  as  is  the  case  in  the  cornification 
of  the  stratum  corneum.  The  cells  flatten  and  form  the  plate-like  structure  of 
which  the  solid  substance  of  the  nail  is  composed.  Thus  the  nail  substance  is 
formed  in  the  proximal  nail  fold.  Over  the  area  termed  the  lunula  (the  whitish 
crescent  at  the  base  of  the  adult  nail)  the  nail  is  pushed  toward  the  tip  of  the  digit 
by  the  development  of  new  nail  substance  in  the  region  of  the  nail  fold.  The  nail 
matrix,  according  to  Bowen,  represents  a  modified  stratum  lucidum  of  the  epider- 
mis.   The  stratum  corneum  of  the  epidermis  for  a  time  completely  covers  the  nail 


THE   HISTOGENESIS    OF    THE   NERVOUS    TISSUES 


309 


matrix  and  is  termed  the  cponychium  (Fig.  296  C).  Later,  this  is  thrown  off  but 
a  portion  persists  during  life  as  the  curved  fold  of  the  epidermis  which  adheres  to. 
the  lunula  of  the  adult  nail.  During  life  the  nail  constantly  grows  at  its  base 
( I  Maximally),  is  shifted  distally  over  the  bed  of  the  corium,  and  projects  at  the  tip 
of  the  digit.     The  corium  distal  to  the  lunula  takes  no  part  in  the  development 


Sole  plate 


Eponychium 


"Nail  matrix 


Nail  fold 


■Nail  bed 


Fig.  296. — Figures  showing  the  development  of  the  nail.  A  and  B,  in  surface  view;  A  in  a  4 
cm.,  B,  in  a  10  cm.  fetus;  C,  longitudinal  section  through  the  nail  anlage  of  a  10  cm.  fetus.  X  24 
(Kolimann's  Handatlas). 

of  the  nail  substance  and  its  surface  of  contact  with  the  nail  is  thrown  into  parallel 
longitudinal  folds.     These  folds  produce  the  longitudinal  ridges  of  the  nails. 

The  nails  of  man  are  the  homologues  of  the  claws  and  hoofs  of  other  mammals.  During 
the  third  month  thickenings  of  the  integument  over  the  distal  ends  of  the  metacarpals  and 
metatarsals  become  prominent.  These  correspond  to  the  touch-pads  on  the  feet  of  clawed 
mammals.    Similar  pads  are  developed  on  the  under  sides  of  the  distal  phalanges. 


THE  HISTOGENESIS  OF  THE  NERVOUS  TISSUES 
The  primitive  anlage  of  the  nervous  system  consists  of  the  thickened  layer  of 
ectoderm  along  the  mid-dorsal  line  of  the  embryo.  This  is  the  neural  plate  (Fig. 
297  A,  B)  which  is  invaginated  to  form  the  neural  groove.  The  edges  of  the 
neural  plate  come  together  and  form  the  neural  tube  (Fig.  297  C,  D).  The 
cranial  portion  of  this  tube  enlarges  and  is  constricted  into  the  three  primary 
vesicles  of  the  brain  (Fig.  306).  Its  caudal  portion  remains  tubular  and  con- 
stitutes the  spinal  cord.     From  the  cells  of  this  tube  and  the  ganglion  crest  con- 


3io 


HISTOGENESIS 


nected  with  it  are  differentiated  the  nervous  tissues,  the  single  exception  being 
the  nerve  cells  and  fibers  of  the  olfactory  epithelium. 

The  Differentiation  of  the  Neural  Tube. — The  cells  of  the  neural  tube  dif- 
ferentiate along  two  lines:  There  are  formed:  (i)  nerve  cells  and  fibers,  in  which 
irritability  and  conductivity  have  become  the  predominant  functions;  (2)  neu- 
roglia cells  and  fibers  which  form  the  supporting  or  skeletal  tissue  peculiar  to  the 
nervous  system.  The  differentiation  of  these  tissues  has  been  studied  by 
Hardesty  in  pig  embryos  (Amer.  Jour.  Anat.,  vol.  3,  1904).  The  wall  of  the 
neural  tube,  consisting  at  first  of  a  single  layer  of  columnar  cells,  becomes  many 


Neural  groove 

Neural  pi  ah 


Neural  qroove 


Neural  plate 


A 


Ectoderm. 


Neural  groove 


-Neural  tube 

Neural  tube       I  X  Neural  cavity 

C  B 

Fig.  297. — Four  sections  showing  the  development  of  the  neural  tube  in  human  embryos:  A,  of  an 
early  embryo  (Keibel);  B,  through  head  of  a  2  mm.  embryo  (Graf  Spee);  C,  neural  tube  of  2  mm.  em- 
bryo (Mall);  D,  neural  tube  of  2.69  mm.  embryo  (Kollmann). 


layered  and  finally  three  zones  are  differentiated  (Fig.  298  A-D.)  When  the  wall 
becomes  many  layered  the  cells  lose  their  sharp  outlines  and  form  a  compact 
cellular  syncytium  (Fig.  298  B).  On  its  outer  and  inner  surfaces  there  is  differ- 
entiated from  the  cytoplasm  an  external  and  internal  limiting  membrane.  In  a 
10  mm.  embryo  the  cellular  strands  of  the  syncytium  are  radially  arranged  and 
directed  nearly  parallel  to  each  other  (Fig.  298  D).  The  nuclei  are  now  so  grouped 
that  there  may  be  distinguished  three  layers:  (1)  an  inner  ependymal  zone  with 
cells  abutting  on  the  internal  limiting  membrane,  their  processes  extending 
peripherally;    (2)  a  middle  mantle  or  nuclear  zone,  and  (3)  an  outer  or  marginal 


THE   HISTOGENESIS    OF   THE    NERVOUS    TISSUES 


311 


zone,  non-cellular,  into  which  nerve  fibers  grow.  The  ependymal  zone  contributes 
cells  for  the  development  of  the  mantle  layer  (Fig.  298  D.)  The  cellular  mantle 
layer  forms  the  gray  substance  of  the  central  nervous  system,  while  the  fibrous 
marginal  layer  constitutes  the  white  substance  of  the  spinal  cord. 


mii 


mle 


mli 


D 


Fig.  298. — Three  stages  in  the  early  development  of  the  neural  tube  showing  the  origin  of  the  syn- 
cytial framework:  .1 ,  from  rabbit  before  the  closure  of  neural  tube;  B,  from  5  mm.  pig  after  closure  of 
tube;  C,  from  a  7  mm.  embryo;  D,  from  a  10  mm.  pig  embryo,  a,  ependymal  layer;  b,  boundary 
between  nuclear  layer  and  marginal  layer;  g,  germinal  cell;  m,  marginal  layer;  mlc,  mli,  external  and 
internal  limiting  membranes;  r,  mantle  or  nuclear  layer;  />,  mesoderm. 


The  primitive  germinal  cells  of  the  neural  tube  divide  by  mitosis  and  give 
rise  to  the  ependymal  cells  of  the  ependymal  zone  and  to  indifferent  cells  of  the 
mantle  layer.  From  these  arise  spongioblasts  and  neuroblasts  (Fig.  299  ).  The 
spongioblasts  are  transformed  into  neuroglia  cells  and  fibers,  which  form  the  sup- 


312 


HISTOGENESIS 


porting  tissue  of  the  central  nervous  system ;  the  neuroblasts  are  primitive  nerve 
cells  and  by  developing  cell  processes  are  converted  into  neurones.  The  neurones 
are  the  structural  units  of  the  nervous  tissue. 


Cells 


rent  Cells 


ndi'fferent 


NeurogliaCells 
Neurobfasts 

Fig.  299. — Diagrams  showing  the  differentiation  of  the  cells  in  the  wall  of  the  neural  tube  and  the  theo- 
retical derivation  of  the  ependymal  cells,  neuroglia  cells  and  neuroblasts  (after  Schaper). 


) 


FlG.  300. — A ,  Transverse  section  through  the  spinal  cord  of  a  chick  embryo  of  the  third  day  showing 
neuraxons  developing  from  neuroblasts  of  the  neural  tube  F  and  from  the  bipolar  ganglion  cells,  d. 
(Cajalj;  B,  Neuroblasts  from  the  spinal  cord  of  a  seventy-two-hour  chick.  The  three  to  the  right  show 
neurofibrils;    C,  incremental  cone. 


THE   HISTOGENESIS    OF    THE    NERVOUS    TISSUES 


313 


The  Differentiation  of  the  Neuroblasts  into  Neurones. — The  nerve  fibers  are 
developed  as  outgrowths  from  the  neuroblasts,  and  a  nerve  cell  with  all  its  pro- 
cesses constitutes  a  neurone  or  cellular  unit  of  the  nervous  system.  The  origin 
of  the  nerve  libers  as  processes  of  the  neuroblasts  is  best  seen  in  the  development 
of  the  root  libers  of  the  spinal  nerves. 

The  Efferent  or  Ventral  Root  Fibers  of  the  Spinal  Nerves. — At  the  end  of 
the  first  month  clusters  of  neuroblasts  separate  themselves  from  the  syncytium 
in  the  mantle  layer  of  the  neural  tube.  The  outline  of  the  neuroblasts  becomes 
pyriform  and  from  the  small  end  of 
the  cell  a  slender  primary  process 
grows  out  (Figs.  300  and  301).  The 
process  becomes  the  axis  cylinder  of 
a  nerve  fiber.  The  primary  processes 
may  course  in  the  marginal  layer  of 
the  neural  tube,  or,  converging,  may 
penetrate  the  marginal  layer  ventro- 
laterally  and  form  the  ventral  roots 
of  the  spinal  nerves.  Similarly,  the 
efferent  fibers  of  the  cerebral  nerves 
grow  out  from  neuroblasts  of  the  brain 
wall.  Within  the  cytoplasm  of  the 
nerve  cells  and  their  primary  processes 
strands  of  fine  fibrils  early  are  differen- 
tiated. These  are  the  neurofibrillce 
and  are  the  conducting  elements  of 
the  neurones.  The  cell  bodies  of  the 
efferent  neurones  soon  become  multi- 
polar by  the  development  of  branched 


Fig.  301. — Transverse  section  of  the  spinal 
cord  from  an  embryo  of  the  fourth  week  showing 
pear-shaped  neuroblasts  giving  rise  to  ventral  root 
fibers  (His  in  Marshall's  Embryology).  XC,  cen- 
tral canal  of  spinal  cord;  XD.  dorsal  root  of  spinal 
nerve;  XI,  nuclei  of  spongioblasts;  XV,  ventral 
motor  root  fibers;  VII",  ventral  funiculi;  XZ, 
neuroblasts. 


secondary  processes,  the  dendrites. 

The  Development  of  the  Spinal  Ganglia  and  Afferent  Neurones  of  the 
Spinal  Cord. — The  ganglion  crest. — After  the  formation  of  the  neural  plate  and 
groove  a  longitudinal  ridge  of  cells  is  differentiated  on  each  side  where  the  ecto- 
derm and  neural  plate  are  continuous  (Fig.  302  A).  This  ridge  of  ectodermal 
cells  is  the  neural  or  ganglion  crest.  When  the  neural  tube  is  formed  and  the 
ectoderm  separates  from  it,  the  cells  of  the  ganglion  crest  overlie  the  neural  tube 
dorso-laterally  (Fig.  302  C).  As  development  continues  they  separate  into  right 
and  left  linear  crests  distinct  from  the  neural  tube,  and  migrate  ventro-laterally 


,14 


HISTOGENESIS 


to  a  position  between  the  neural  tube  and  myotomes.  In  this  position  the 
ganglion  crest  forms  a  band  of  cells  extending  the  whole  length  of  the  spinal  cord 
and  as  far  cranially  as  the  otic  vesicles.  At  regular  intervals  in  its  course  along 
the  spinal  cord  the  proliferating  cells  of  the  crest  give  rise  to  enlargements,  the 
spinal  ganglia  (Fig.  340).  The  spinal  ganglia  are  segmentally  arranged  and  con- 
nected at  first  by  bridges  of  cells  which  later  disappear.  In  the  hind-brain  region 
certain  ganglia  of  the  cerebral  nerves  develop  from  the  crest  but  are  not  seg- 
mentally arranged. 


tab     *  C 


Fig.  302. — Three  stages  in  the  development  of   the  ganglion  crest  in  human   embryos   (after  von 
Lenhossek  in  Cajal).     a,  ectoderm;  b,  neural  tube;  c,  mesodermal  segment;   G,  ganglioblasts. 


The  Differentiation  of  the  Afferent  Neurones. — The  cells  of  the  spinal  ganglia 
differentiate  into  (1)  ganglion  cells  and  (2)  supporting  cells,  groups  which  are 
1  om parable  to  the  neuroblasts  and  spongioblasts  of  the  neural  tube.  The  neuro- 
blasts of  the  ganglia  become  fusiform,  develop  a  primary  process  at  either  pole 
and  thus  these  neurones  are  of  the  bipolar  type.  The  centrally  directed  processes 
of  the  ganglion  cells  converge  and  by  elongation  form  the  dorsal  roots.  They 
penetrate  the  dorso-lateral  wall  of  the  neural  tube,  bifurcate  and  course  cranially 
and  caudally  in  the  marginal  layer  of  the  spinal  cord  (Fig.  300,  d).  By  means  of 
branched  processes  they  anastomose  with  the  neurones  of  the  mantle  layer.  The 
peripheral  processes  of  the  ganglion  cells  as  the  dorsal  spinal  roots  join  the  ventral 


THE   HISTOGENESIS    <>F   THE   NERVOUS    IISSl  ES 


315 


roots  and,  together  with  them,  constitute  the  trunks  of  the  spinal  nerves  (Fig. 

307)- 

The  Differentiation  of  the  Unipolar  Ganglion  Cells.     At  first  bipolar,  the 

majority  of  the  ganglion  cells  become  unipolar  either  by  the  unilateral  growth 
of  the  cell  body  or  by  the  bifurcation  of  a  single  primary  process.  In  the  first  case, 
if  the  cytoplasm  and  nucleus  take  up  an  eccentric  position,  the  two  processes 
unite  in  a  single  slender  connection  with  the  cell  body  (Fig.  303).  The  ganglion 
cell,  having  one  process,  is  now  unipolar  and  its  process  is  T-shaped.  Many  of  the 
bipolar  ganglion  cells  persist  in  the  adult,  and  others  develop  several  secondary- 
processes  and  thus  become  multipolar 
in  form.  In  addition  to  forming  the 
spinal  ganglion  cells,  neuroblasts  of 
the  ganglion  crest  are  believed  to 
migrate  ventrally  and  form  the  sym- 
pathetic ganglia  (Fig.  307).  C- 

Tfie  Neurone  Theory. — The  above  ac- 
count of  the  development  of  the  nerve  fibers 
is  the  one  generally  accepted  at  the  present 
time.  It  assumes  that  the  axis  cylinders  of 
all  nerve  fibers  are  formed  as  outgrowths, 
each  from  a  single  cell,  an  hypothesis  first 
promulgated  by  His.  The  embryological 
evidence  is  supported  by  experiment.  It 
has  long  been  known  from  the  work  of 
Waller  that  if  nerves  are  severed,  the  fibers 
distal  to  the  point  of  section,  and  thus  iso- 
lated from  their  nerve  cells,  will  degenerate; 
also,  that  regeneration  will  take  place  from 
the  central  stumps  of  cut  nerves,  the  fibers 
of  which  are  still  connected  with  their  cells. 
More  recently  Harrison  (Amer.  Jour.  Anat., 

vol.  5, 1906)  experimenting  on  amphibian  lame  has  shown  (1)  that  no  peripheral  nerves  develop 
if  the  neural  tube  and  crest  are  removed;  (2)  that  isolated  ganglion  cells  growing  in  clotted 
lymph  will  give  rise  to  long  axis  cylinder  processes  in  the  course  of  four  or  five  hours. 

A  second  theory,  supported  by  Schwann,  Balfour,  Dohrn  and  Bethe,  assumes  that  the 
nerve  fibers  are  in  part  differentiated  from  a  chain  of  cells,  so  that  the  neurone  would  represent 
a  multicellular,  not  a  unicellular  structure.  Apathy  and  O.  Schulze  modified  this  cell-chain 
tlieory  by  assuming  that  the  nerve  fibers  differentiate  in  a  syncytium  which  intervenes  between 
the  neural  tube  and  the  peripheral  end  organs.  Held  further  modified  this  theory  by  assuming 
that  the  proximal  portions  of  the  nerve  fibers  are  derived  from  the  neuroblasts  and  ganglion 
cells  and  that  these  grow  into  a  syncytium  which  by  differentiation  gives  rise  to  the  peripheral 
portion  of  the  fiber.  This  theory  accords  with  the  experiments  of  Bethe  who  found  that  in  the 
peripheral  portions  of  severed  nerves,  functional  nerve  fibers  were  regenerated  in  young 
animals. 


Fig.  303. — A  portion  of  a  spinal  ganglion  from  a 
human  embryo  of  44  mm.  Golgi  method  (Cajal). 


316  HISTOGENESIS 

The  Differentiation  of  the  Supporting  Cells  of  the  Ganglia  and  Neural  Tube. 

— The  supporting  cells  of  the  spinal  ganglia  at  first  form  a  syncytium  in  the 
meshes  of  which  are  found  the  neuroblasts.  They  differentiate  (i)  into  flattened 
capsule  cells  which  form  capsules  about  the  ganglion  cells  and  (2)  into  sheath  cells 
which  ensheath  the  axis  cylinder  processes  and  are  continuous  with  the  capsules 
of  the  ganglion.  It  is  probable  that  many  of  the  sheath  cells  migrate  peripherally 
along  with  the  developing  nerve  fibers  (Harrison).  They  are  at  first  spindle- 
shaped  and,  as  primary  sheaths,  enclose  bundles  of  nerve  fibers.  Later,  by  the 
proliferation  of  the  sheath  cells  the  bundles  are  separated  into  single  fibers,  each 
with  its  sheath  (of  Schwann),  or  neurilemma.  Each  sheath  cell  forms  a  segment 
of  the  neurilemma,  the  limits  of  adjacent  sheath  cells  being  indicated  by  constric- 
tions, the  nodes  of  Ranvier. 

The  Myelin  or  Medullary  Sheath. — During  the  fourth  month  an  inner  myelin 
sheath  appears  about  many  nerve  fibers.  This  consists  of  a  spongy  framework  of 
neurokeratin  in  the  interstices  of  which  a  fatty  substance,  myelin,  is  deposited. 
The  origin  of  the  myelin  sheath  is  in  doubt.  By  some  it  is  believed  to  be  a  differ- 
entiation of  the  neurilemma,  the  myelin  being  deposited  in  the  substance  of  the 
nucleated  sheath  cell.  By  others  the  myelin  is  regarded  as  a  product  of 
the  axis  cylinder.  Its  integrity  is  dependent  at  least  upon  the  nerve  cell 
and  axis  cylinder,  for,  when  a  nerve  is  cut,  the  myelin  very  soon  shows 
degenerative  changes. 

In  the  central  nervous  system  there  is  no  distinct  neurilemma  sheath  investing 
the  fibers.  Sheath  cells  are  said  to  be  present  and  most  numerous  during  the 
period  when  myelin  is  developed.  Hardesty  derives  the  sheath  cells  in  the 
central  nervous  system  of  the  pig  from  a  portion  of  the  supporting  cells, 
or  spongioblasts,  of  the  neural  tube,  and  finds  that  these  cells  give  rise  to  the 
myelin  of  the  fibers. 

Those  fibers  which  are  first  functional  receive  their  myelin  sheaths  first. 
The  development  of  myelin  is  only  completed  between  the  fifteenth  and  twentieth 
year  (Westphal).  Many  of  the  peripheral  fibers,  especially  those  of  the  sympa- 
thetic system,  remain  non-medullated  and  supplied  only  with  a  neurilemma  sheath. 
The  medullated  fibers,  those  with  a  myelin  sheath,  have  a  glistening  white  ap- 
pearance and  give  the  characteristic  color  to  the  white  substance  of  the  central 
nervous  system  and  to  the  peripheral  nerves.  Ranson  (Amer.  Jour.  Anat.,  vol. 
12,  p.  67)  has  shown  that  large  numbers  of  non-medullated  fibers  occur  in  the 
peripheral  nerves  and  spinal  cord  of  adult  mammals  and  man.  Those  found  in 
the  spinal  nerves  arise  from  the  small  cells  of  the  spinal  ganglia. 


THE    HISTOGENESIS    OF    THE    NERVOUS    TISSIKS 


317 


The  Development  of  the  Neuroglia  Cells  and  Fibers. — The  spongioblasts 
of  the  neural  tube  (seep.  311)  differentiate  into  the  supporting  tissue  of  the  central 
nervous  system.  This  includes  the  cpendymal  cells,  which  line  the  neural  cavity, 
forming  one  of  the  primary  layers  of  the  neural  tube,  neuroglia  cells  and  their 
fibers. 

We  have  described  how  the  strands  of  the  syncytium  formed  by  the  spongio- 
blasts become  arranged  radially  in  the  neural  tube  of  early  embryos  (Fig.  298  D). 
As  the  wall  of  the  neural  tube  thickens,  the  strands  elongate  pari  passu  and  form  a 


Fig.  304. — Ependvmal  cells  from  the  neural  tube  of  chick  embryos:   A,  of  first  day;   B,  of  third  day. 

Golgi  method  (Cajal). 


radiating  branched  framework  (Fig.  304  A,  B).  The  group  of  spongioblasts 
which  line  the  neural  cavity  constitutes  the  ependvmal  layer.  Processes  from 
these  cells  radiate  and  extend  through  the  whole  thickness  of  the  neural  tube  to 
its  periphery.  The  cell  bodies  are  columnar  and  persist  as  the  lining  of  the 
central  canal  and  ventricles  of  the  spinal  cord  and  brain  (Fig.  305). 

Near  the  median  line  of  the  spinal  cord,  both  dorsally  and  ventrally, 
the  supporting  tissue  retains  its  primitive  ependvmal  structure  in  the  adult. 
Elsewhere  the  supporting  framework  is  differentiated  into  neuroglia  cells  and 
fibers.     The  neuroglia  cells  form  part  of  the  spongioblasts  syncytium  and  are 


3i8 


HISTOGENESIS 


scattered  through  the  mantle  and  marginal  layers  of  the  neural  tube.  By  pro- 
liferation they  increase  in  numbers  and  their  form  depends  upon  the  pressure 
of  the  nerve  cells  and  fibers  which  develop  around  them. 

Neuroglia  fibers  are  differentiated  from  the  cytoplasm  and  cytoplasmic 


Fig.  305. — Ependymal  cells  of  the  lumbar  cord  from  a  human  embryo  of  44  mm.  Golgi  method 
(Cajalj.  A,  floor  plate;  B,  central  canal;  C,  line  of  future  fusion  of  walls  of  neural  cavity;  D, 
neuroglia  cells  and  fibers. 


processes  of  the  neuroglia  cells,  and  as  the  latter  primarily  form  a  syncytium,  the 
neuroglia  fibers  may  extend  from  cell  to  cell.  The  neuroglia  fibers  develop  late 
in  fetal  life  and  undergo  a  chemical  transformation  into  neurokeratin,  the  same 
substance  which  is  found  in  the  sheaths  of  medullated  fibers. 


CHAPTER  XI 

THE  MORPHOGENESIS  OF  THE  CENTRAL  NERVOUS  SYSTEM 

In  discussing  the  histogenesis  of  the  nervous  tissue  we  have  described  the 
early  development  of  the  neural  tube,  as  an  infolding  of  the  neural  plate  and  a 
closure  of  the  neural  groove  (Fig.  297).  The  groove  begins  to  close  along  the  mid- 
dorsal  line  near  the  middle  of  the  body  in  embryos  of  2  mm.  and  the  closure 


Mesencephalon 

Rhombencephalon 
Mycloicephalon 

Amnion  (cut) 


Mesodermal  segment  14 


Neural  tube  (not  closed) 


Prosencephalon 


Stomodaum 


Amnion  (cut) 


Yolk-sac 


Body-stalk 


Fig.  306. — Human  embryo  of  2.4  mm.  showing  neural  tube  closing  and  the  brain  vesicles 

(after  Kollmann). 

extends  both  cranially  and  caudally  (Fig.  306) .  Until  the  end  of  the  third  week 
there  still  persists  an  opening  at  either  end  of  the  neural  tube,  somewhat  dorsad. 
These  openings  are  the  neuroporcs.  Before  the  closure  of  the  neuropores,  in 
embryos  of  2  to  2.5  mm.  the  cranial  end  of  the  neural  tube  has  enlarged  and  is 

319 


120 


THE   MORPHOGENESIS    OF   THE   CENTRAL   NERVOUS    SYSTEM 


constricted  at  two  points  to  form  the  three  primary  brain  vesicles.  The  caudal 
two-thirds  of  the  neural  tube,  which  remains  of  smaller  diameter,  constitutes  the 
uiil age  of  the  spinal  cord. 

THE  SPINAL  CORD 

The  spinal  portion  of  the  neural  tube  is  at  first  nearly  straight,  but  is  bent 
with  the  flexure  of  the  embryo  into  a  curve  which  is  convex  dorsally.  Its  wall 
gradually  thickens  during  the  first  month  and  the  diameter  of  its  cavity  is  di- 


Neura.1  cavity 


Marginal  layer 


Dorsal   root 
Ependymal  layer 
Spinal  ganglion 


Mantle 
layer 


Dorsal 
ramus 


Ventral 
root 


Nerve  trunK  Sympathetic  ganglion 

Fig.  307. — Transverse  section  through  a  10  mm.  embryo  at  the  level  of  the  arm  buds  showing  the 
spinal  cord  and  a  spinal  nerve  of  right  side.     X  44. 

minished  from  side  to  side.  By  the  end  of  the  first  month  three  layers  have  been 
developed  in  its  wall  as  described  in  Chapter  X,  p.  310  (Fig.  307).  These  layers 
are  the  inner  ependymal  layer  which  forms  a  narrow  zone  about  the  neural  cavity; 
the  middle  mantle  layer \  cellular,  and  the  outer  marginal  layer,  fibrous. 

The  Ependymal  Layer  is  differentiated  into  a  dorsal  roof  plate  and  a  ventral 
floor  plate  (Fig.  308).  Laterally,  its  proliferating  cells  contribute  neuroblasts  and 
neuroglia  cells  to  the  mantle  layer.     The  proliferation  of  cells  ceases  first  in  the 


THE    SPINAL    CORD 


321 


Centra]  portion  of  the  layer,  which  is  thus  narrower  than  the  dorsal  portion  in 
10  to  20  mm.  embryos  (Figs.  307  and  308).  Consequently,  the  ventral  portion 
of  the  mantle  layer  is  differentiated  first.  The  neural  cavity  is  at  first  somewhat 
rhomboidal  in  transverse  section,  wider  dorsally  than  ventrally.  Its  lateral  angle 
forms  the  sulcus  litnitans  which  marks  the  subdivision  of  the  lateral  walls  of  the 
neural  tube  into  the  dorsal  alar  plate  and  ventral  basal  plate.  When  the  ependy- 
mal  layer  ceases  to  contribute  new  cells  to  the  mantle  laser  its  walls  are  approxi- 
mated dorsally.  As  a  result,  in  20  mm.  embryos  the  neural  cavity  is  wider  ven- 
trally (Fig.  208).  In  the  next  stage,  34  mm.,  these  walls  fuse  and  the  dorsal 
portion  of  the  neural  cavity  is  obliterated  (Fig.  309).     In  a  65  mm.  embryo  the 


Hoof  plate 


Post  .funiculus 


Neural  cavity 


Marginal  layer 


EpendyHial  /-aye, 


Ant.  median  fissurt 


Floor  plate 
Fig.  308. — Transverse  section  of  the  spinal  cord  from  a  20  mm.  embryo.     X  44. 


persisting  cavity  is  becoming  rounded  (Fig.  310).  It  forms  the  central  canal  of 
the  adult  spinal  cord.  The  cells  lining  the  central  canal  are  ependymal  cells 
proper.  Those  in  the  floor  of  the  canal  form  the  persistent  floor  plate.  Their 
fibers  extend  ventrally  to  the  surface  of  the  cord  in  the  depression  of  the  ventral 
median  fissure. 

When  the  right  and  left  walls  of  the  ependymal  layer  fuse,  the  ependymal 
cells  of  the  roof  plate  no  longer  radiate,  but  form  a  medium  septum  (Fig.  309). 
Later,  as  the  marginal  layers  of  either  side  thicken  and  are  approximated  the 
median  septum  is  extended  dorsally.  Thus  the  roof  plate  is  converted  into  part 
of  the  dorsal  median  septum  of  the  adult  spinal  cord  (Fig.  310). 

The  Mantle  Layer,  as  we  have  seen,  is  contributed  to  by  the  proliferating 


32: 


THE   MORPHOGENESIS    OF   THE   CENTRAL   NERVOUS    SYSTEM 


cells  of  the  ependymal  layer.  A  ventro-lateral  thickening  first  becomes  promi- 
nent in  embryos  of  10  to  15  mm.  (Fig.  307).  This  is  the  ventral  gray  column,  or 
horn,  which  in  later  stages  is  subdivided,  forming  also  a  lateral  gray  column  (Fig. 


Dorsal  funiculus 


Post-median  Septum 

Lat.funiculus 
Central  canal 


Ant.column 


Dura  mater 


[£$,  Spinal 
*j||r  gang ft'on 


Ventral  funiculus 

Fig.  309. — Transverse  section  of  the  spinal  cord  from  a  34  mm.  embryo,  showing  also  the  spinal  ganglion 
and  dura  mater  on  the  left  side.     X  44. 


Post  medium  s  ep  tu  m 
Post  root 


Post,  col  urn 


Lat,  fun  ic 


Ant.  column 


Fascic.  gracilis 


Fascic. 


bstantla  gelatmosa 

c 


..tral  , 
canal 


Lat. 

Column 


Ant.  funiculus     -J  ^Ant.median  fissure 

Fig.  310. — Transverse  section  of  the  spinal  cord  from  a  65  mm.  embryo.     X  44. 


310).  It  is  a  derivative  of  the  basal  plate.  In  embryos  of  20  mm.  a  dorso-lateral 
thickening  of  the  mantle  layer  is  seen,  the  cells  of  which  constitute  the  dorsal 
gray  column  or  horn  (Figs.  208  and  209) ;  about  these  cells  end  the  collaterals  of 


I  111      SPINAL    (OKI) 


323 


the  dorsal  roof  libers.  The  cells  of  the  dorsal  gray  column  thus  form  terminal 
nuclei  for  the  afferent  spinal  nerve  fibers  and  they  are  derivatives  of  the  alar 
plate  of  the  cord.  Dorsal  and  ventral  to  the  central  canal  the  marginal  layer 
forms  the  dorsal  and  ventral  gray  commissures.  In  the  ventral  floor  plate  nerve 
fibers  cross  from  both  sides  of  the  cord  and  form  the  anterior  white  commissure. 

The  Marginal  Layer  is  composed  primarily  of  a  framework  of  neuroglia  and 
ependymal  ceH  processes.  Into  this  framework  grow  the  axis  cylinder  processes 
of  nerve  cells,  so  that  the  thickening  of  this  layer  is  due  to  the  increasing  number 
of  nerve  fibers  contributed  to  it  by  ganglion  cells  and  neuroblasts  located  outside 
of  it.  When  their  myelin  develops,  these  fibers  form  the  white  substance  of  the 
spinal  cord.  The  fibers  have  three  sources  (Fig.  342) :  (1)  they  may  arise  from 
the  spinal  ganglion  cells,  entering  as  dorsal  root  fibers  and  coursing  cranially  and 
caudally  in  the  marginal  layer;  (2)  they  may  arise  from  neuroblasts  in  the  mantle 
layer  of  the  spinal  cord  (a)  as  fibers  which  connect  adjacent  nuclei  of  the  cord 
(fasciculi  proprii  or  ground  bundles) ;  (b)  as  fibers  which  extend  cranially  to  the 
brain;  (3)  they  may  arise  from  neuroblasts  of  the  brain  (a)  as  long  descending 
cerebrospinal  tracts  from  the  cortex  of  the  cerebrum;  (b)  as  descending  tracts 
from  the  brain  stem. 

Of  these  fiber  tracts  (1)  and  (2  a)  appear  during  the  first  month;  (2  b)  and 
(3  b)  during  the  third  month;   (3  a)  at  the  end  of  the  fifth  month. 

The  dorsal  root  fibers  from  the  spinal  ganglion  cells  entering  the  cord  dorso- 
laterally  subdivide  the  white  substance  of  the  marginal  layer  into  a  dorsal  funi- 
culus and  lateral  funiculus.  The  lateral  funiculus  is  marked  off  by  the  ventral 
root  fibers  from  the  ventral  funiculus  (Fig.  309).  The  ventral  root  fibers,  as  we 
have  seen,  take  their  origin  from  the  neuroblasts  of  the  ventral  gray  column  in  the 
mantle  layer.     They  are  thus  derivatives  of  the  basal  plate. 

The  dorsal  funiculus  is  formed  chiefly  by  the  dorsal  root  fibers  of  the  ganglion 
cells  and  is  subdivided  into  two  distinct  bundles,  the  fasciculus  gracilis,  median, 
and  the  fasciculus  cuneatus,  lateral  in  position.  The  dorsal  funiculi  are  separated 
only  by  the  dorsal  median  septum  (Fig.  310). 

The  lateral  and  ventral  funiculi  are  composed  of  fasciculi  proprii  or  ground 
bundles,  originating  in  the  spinal  cord,  of  ascending  tracts  from  the  cord  to  the 
brain,  and  of  the  descending  fiber  tracts  from  the  brain.  The  fibers  of  these 
fasciculi  intermingle  and  the  fasciculi  are  thus  without  sharp  boundaries.  The  floor 
plate  of  ependymal  cells  lags  behind  in  its  development,  and  as  it  is  interposed 
between  the  thickening  right  and  left  walls  of  the  ventral  funiculi,  these  do  not 
meet  and  the  ventral  median  fissure  is  produced  (compare  Figs.  307  and  310). 


324 


THE   MORPHOGENESIS    OF   THE   CENTRAL  NERVOUS    SYSTEM 


Cervical  enlargement 


Lumbar  enlargement 


The  development  of  myelin  in  the  nerve  fibers  of  the  cord  begins  in  the  fifth  month  of 
fetal  life  and  is  completed  between  the  fifteenth  and  twentieth  years  (Flechsig,  Bechterew). 
Mvelin  appears  first  in  the  root  fibers  of  the  spinal  nerves  and  in  those  of  the  ventral  commissure, 
next  in  the  ground  bundles,  and  dorsal  funiculi.  The  cerebrospinal  (pyramidal)  fasciculi  are 
the  last  in  which  myelin  is  developed;  they  are  myelinated  during  the  first  and  second  years. 
As  myelin  appears  in  the  various  fiber  tracts  at  different  periods,  this  fact  has  been  utilized  in 
tracing  the  extent  and  origin  of  the  various  fasciculi  in  the  central  nervous  system. 

The  Cervical  and  Lumbar  Enlargements. — At  the  levels  of  the  two  nerve 
plexuses  supplying  the  upper  and  lower  extremities  the  size  of  the  spinal  cord  is 

increased.  As  the  fibers  to  the  mus- 
cles of  the  extremities  arise  from  nerve 
cells  in  the  ventral  gray  column,  the 
number  of  these  cells  and  the  mass  of 
the  gray  substance  is  increased;  also 
larger  numbers  of  fibers  enter  the  cord 
from  the  integument  of  the  limbs,  so 
that  there  are  larger  numbers  of  cells 
about  which  sensory  fibers  terminate. 
There  is  formed  consequently  at  the 
level  of  the  origin  of  the  nerves  of  the 
brachial  plexus  the  cervical  enlarge- 
ment, opposite  the  origins  of  the 
nerves  of  the  lumbo-sacral  plexus  the 
lumbar  enlargement  (Fig.  311). 

At  the  caudal  end  of  the  neural 

tube  in  an  11  cm.  fetus  an  epithelial 

sac  is  formed  which  is  adherent  to  the 

integument.     Cranial  to  the  sac  the  central  canal  is  obliterated  and  this  part  of 

the  neural  tube  forms  the  filum  terminate.     The  caudal  end  of  the  central  canal 

is  irregularly  expanded  and  is  known  as  the  terminal  ventricle. 

The  vertebral  column  during  and  after  the  third  month  grows  faster  than  the 
spinal  cord.  As  the  cord  is  fixed  to  the  brain  it  is  carried  cranially  with  reference 
to  the  vertebrae,  and  with  it  shift  the  roots  and  ganglia  of  the  spinal  nerves.  In 
the  adult  the  origin  of  the  coccygeal  nerves  is  opposite  the  first  lumbar  vertebra 
and  the  nerves  course  obliquely  downward  nearly  parallel  to  the  spinal  cord.  As 
the  neural  tube  is  drawn  cranially  and  its  caudal  tip  is  attached  to  the  coccyx,  its 
caudal  portion  is  stretched  into  the  slender  solid  cord  known  as  the  filum  terminate. 


Fig.  311 . — Dissection  of  the  brain  and  cord  of 
a  three  months'  fetus,  showing  the  cervical  and 
lumbar  enlargements  (after  Ko Hiker  in  Marshall). 
c,  cerebellum;  h,  cerebrum;  m,  mid-brain.  Nat- 
ural size. 


THE    BRAIN 


325 


The  obliquely  coursing  spinal  nerves  with  the  lilum  terminale  constitute  the 
Cauda  equina. 

THE  BRAIN 
We  have  seen  that  in  embryos  of  2  to  2.5  mm.  the  neural  tube  is  nearly 
straight,  but  that  its  cranial  end  is  enlarged  to  form  the  anlage  of  the  brain.  The 
appearance  of  two  constrictions  in  the  wall  of  the  anlage  subdivides  it  into  the 
three  primary  brain  vesicles,  the  fore-brain  or  prosencephalon,  mid-brain  or 
mesencephalon,  and  hind-brain  or  rhombencephalon  (Fig.  306). 


Anterior  neuropore 

iPallium  of  telencephalon 


Palli 


Corpus  striatum 


Diencephalon 

/Interior  neuropore 

Mes  en  cephalon 
Isthmus 


Mese  n  c  ephalon 


Cephalic 
'     flexure 


Optic  recess 

Future  pontine 
Rhombencephalon       flexure 


-  Rhombencephalon 


A  D 

Fig.  312. — Reconstructions  of  the  brain  of  a  3.2  mm.  human  embryo.     A,  lateral  surface;   B,  sectioned 
in  the  median  sagittal  plane  (after  His). 


In  embryos  of  3.2  mm.,  estimated  age  three  weeks,  three  important  changes 
have  taken  place  (Fig.  312  A,  B) :  (1)  the  end  of  the  neural  tube  is  bent  sharply 
in  the  mid-brain  region  so  that  the  axis  of  the  fore-brain  now  forms  a  right  angle 
with  the  axis  of  the  hind-brain.  This  bend  is  the  cephalic  flexure;  (2)  the  fore- 
brain  shows  indication  dorsally  of  a  fold  the  margo  thalamicus  which  subdivides 
it  into  the  telencepalon  and  the  diencephalon;  (3)  the  lateral  wall  of  the  fore-brain 
shows  a  distinct  evagination,  the  optic  vesicle,  which  projects  laterally  and  caudad. 
A  ventral  bulging  of  the  wall  of  the  hind-brain  indicates  the  position  of  the  future 
pontine  flexure. 

In  embryos  of  7  mm.  (four  weeks)  the  neuropores  have  closed.     The  ceph- 


326 


THE    MORPHOGENESIS    OF    THE    CENTRAL   NERVOUS    SYSTEM 


alic  flexure,  now  more  marked,  forms  an  acute  angle  and  the  pontine  flexure, 
just  indicated  in  the  previous  stage,  is  now  a  prominent  ventral  bend  in  the 
ventro-lateral  walls  of  the  hind-brain  (Fig.  313  A,  B).  This  flexure  forms  the 
boundary  line  which  subdivides  the  rhombencephalon  into  a  cranial  portion,  the 
metencephalon,  and  into  a  caudal  portion,  the  myelencephalon.  At  a  third  bend 
the  whole  brain  is  flexed  ventrally  at  an  angle  with  the  axis  of  the  spinal  cord. 
This  bend  is  the  cervical  flexure  and  is  the  line  of  demarcation  between  the  brain 
and  spinal  cord.  The  telencephalon  and  diencephalon  are  more  distinctly  sub- 
divided, and  the  invaginated  optic  vesicle  forms  the  optic  cup  attached  to  the 
brain  wall  by  a  hollow  stalk,  which  later  becomes  the  optic  nerve.     The  walls  of 


Diencephalon 


Mesencephalon 


Pallium 


Ceph.  fie 


Myelencephalon 


Metencephalon 

Corpus  striatum 
Optic  recess 
Hypothalamus 


Mesencephalon 


Isthmus 


Cerebellum 


Fig.  313. — Reconstructions  of  the  brain  of  a  7  mm.  human  embryo.     A,  lateral  view;   B,  in  median 
sagittal    section    (His).     Ceph.    flex.,    cephalic    flexure. 


the  brain  show  a  distinct  differentiation  in  certain  regions.  This  is  especially 
marked  in  the  myelencephalon,  which  has  a  thicker  ventro-lateral  wall  and  thinner 
dorsal  wall. 

Embryos  of  10.2  mm.  show  the  structure  of  the  brain  at  the  beginning  of  the 
second  month  (Figs.  323  and  326).  In  Fig.  341  the  external  form  of  the  brain 
is  seen  with  the  origins  of  the  cerebral  nerves.  It  will  be  noted  that,  with  the 
exception  of  the  first  four  (the  olfactory,  optic,  oculomotor  and  trochlear),  the 
cerebral  nerves  take  their  superficial  origin  from  the  myelencephalon.  The  five 
brain  regions  are  now  sharply  differentiated  externally  but  the  boundary  line 
between  the  telencephalon  and  diencephalon  is  still  indistinct.     The  telencephalon 


THE    BRAIN 


327 


consists  in  paired  lateral  outgrowths,  the  anlagcs  of  the  cerebral  hemispheres  and 
rhinencephalon. 

The  cephalic  flexure  forms  a  very  acute  angle  and,  as  a  result,  the  long  axis 
of  the  fore-brain  is  nearly  parallel  to  that  of  the  hind-brain.  The  oculomotor 
nerve  takes  its  origin  from  the  ventral  wall  of  the  mesencephalon.  Dorsally 
there  is  a  constriction,  the  isthmus,  between  the  mesencephalon  and  meten- 
cephalon,  and  here  the  fibers  of  the  trochlear  nerve  take  their  superficial  origin. 
The  dorsal  wall  of  the  myelencephalon  is  an  exceedingly  thin  ependymal  layer, 


Cerebral  peduncles 


Hypothalamus 
EpUhalamus 
Thalamus 
Diencephalon 
(I  iiter-brain) 


Cerebral  aqueduct 

'■   Mesencephalon 
{Mid-brain) 


Rhombencephalic 
isthmus 


-  .Lamina  Corpi.s 
Rhinencephalon ''terminalis  striatum 
(Olfactory-brain) 


Fig.  314. — Brain  of  a  13.6  mm.  human  embryo  in  median  sagittal  section  (after  His  from  Sobotta's 
Atlas  of  Anatomy).     1,  optic  recess;  2,  ridge  formed  by  optic  chiasma,  3;  4,  infundibular  recess. 


the  tela  chorioidca.     The  ventro-lateral  walls  of  this  same  region  on  the  other  hand 
are  very  thick. 

A  median  sagittal  section  of  a  brain  at  a  somewhat  later  stage  shows  the 
cervical,  pontine  and  cephalic  flexures  well  marked  (Fig.  314).  The  thin  dorso- 
lateral roof  of  the  myelencephalon  has  been  removed.  The  telencephalon  is  a 
paired  structure.  In  the  figure  its  right  half  projects  cranial  to  the  primitive 
median  wall  of  the  fore-brain  which  persists  as  the  lamina  terminalis  (see  Fig.  324). 
The  floor  of  the  telencephalon  is  greatly  thickened  caudally  as  the  anlage  of  the 
corpus  striatum.  A  slight  evagination  of  the  ventral  wall  of  the  telencephalon 
just  cranial  to  the  corpus  striatum  marks  the  anlage  of  the  rhinencephalon.  The 
remaining  portion  of  the  telencephalon  forms  the  pallium  or  cortex  of  the  cerebral 
hemispheres.     The  paired  cavities  of  the  telencephalon  are  the  lateral  (second) 


328  THE   MORPHOGENESIS    OF   THE   CENTRAL   NERVOUS    SYSTEM 

ventricles  and  these  communicate  through  the  interventricular  foramina  (Monroi) 
with  the  cavity  of  the  diencephalon,  the  third  ventricle.  The  cavities  of  the  ol- 
factory lobes  communicate  during  fetal  life  with  the  lateral  ventricles  and  were 
formerly  called  the  first  ventricles. 

The  crossing  of  a  portion  of  the  optic  nerve  fibers  in  the  floor  of  the  brain 
forms  the  optic  chiasma  and  this,  with  the  transverse  ridge  produced  by  it  inter- 
nally, is  taken  as  the  ventral  boundary  line  between  the  telencephalon  and  dien- 
cephalon (Fig.  314).  A  dorsal  depression  separates  the  latter  from  the  mesenceph- 
alon. The  lateral  wall  of  the  diencephalon  is  thickened  to  form  the  thalamus, 
the  caudal  and  lateral  portion  of  which  constitutes  the  metathalamns .  From  the 
metathalamus  are  derived  the  geniculate  bodies.  In  the  median  dorsal  wall,  near 
the  caudal  boundary  line  of  the  diencephalon,  an  outpocketing  begins  to  appear 
in  embryos  of  five  weeks  (Fig.  314).  This  is  the  epithalamus  which  later  gives  rise 
to  the  pineal  body,  or  epiphysis. 

The  thalamus  is  marked  of  from  the  more  ventral  portion  of  the  diencephalic 
wall,  termed  the  hypothalamus  by  the  obliquely  directed  sulcus  hypothalamicus. 
Cranial  to  the  optic  chiasma  is  the  optic  recess,  regarded  as  belonging  to  the 
telencephalon.  Caudal  to  it  is  the  pouch-like  infundibulum,  an  extension  from 
which  during  the  fourth  week  forms  the  posterior  lobe  of  the  hypophysis.  Caudal 
to  the  infundibulum  the  floor  of  the  diencephalon  forms  the  tuber  cinereum  and 
the  mammillary  recess;  the  walls  of  the  latter  thicken  later  and  give  rise  to  the 
mammillary  bodies.  An  oblique  transverse  section  through  the  telencephalon 
and  hypothalamic  portion  of  the  diencephalon  (Fig.  325),  shows  the  relation  of 
the  optic  recess  to  the  optic  stalk,  the  infundibulum  and  Rathke's  pocket,  and  the 
extension  of  the  third  ventricle,  the  proper  cavity  of  diencephalon,  into  the  telen- 
cephalon between  the  corpora  striata. 

The  mesencephalon  in  13.6  mm.  embryos  (Fig.  314)  is  distinctly  marked  off 
from  the  metencephalon  by  the  constriction  which  is  termed  the  isthmus.  Dorso- 
laterally  thickenings  form  the  corpora  quadrigemina.  Ventrally,  the  mesenceph- 
alic wall  is  thickened  to  form  the  tegmentum  amd  crura  cerebri.  In  the  tegmen- 
tum are  located  the  nuclei  of  origin  for  the  oculomotor  and  trochlear  nerves.  The 
former,  as  we  have  seen,  takes  its  superficial  origin  ventrally,  while  the  trochlear 
nerve  fibers  bend  dorsad,  cross  at  the  isthmus  and  emerge  on  the  opposite  side. 
As  the  walls  of  the  mesencephalon  thicken,  its  cavity  later  is  narrowed  to  a  canal, 
the  cerebral  aqueduct  (of  Sylvius). 

The  walls  of  the  metencephalon  are  thickened  dorsally  and  laterally  to  form 


THE   BRAIN 


329 


the  anlage  of  the  cerebellum.     Its  thickened  ventral  wall  becomes  the  pons 
(Varolii).     Its  cavity  constitutes  the  cranial  portion  of  the  fourth  ventricle. 

The  caudal  border  of  the  pons  is  taken  as  the  ventral  boundary  line  between 
the  metencephalon  and  myclcnccphalon.  The  myelencephalon  forms  the  medulla 
oblongata.  Its  dorsal  wall  is  a  thin  non-nervous  ependymal  layer,  which  later 
becomes   the  posterior  medullary  velum.     From   its   thickened   ventro-lateral 


ib         mb     is  hb  IVv 


~ab 


in  m    pf 


ol        0       pf    0 


mb  hb 


Fig.  315. — Brains  of  human  embryos,  from  reconstructions  by  His:  A,  brain  from  fifteen-day  em- 
bryo; B,  from  three-and-a-half-week,  embryo;  C,  from  seven-and-a-half-week  fetus;  fb,  ib,  mb,  hb,  ab, 
fore-,  inter-,  mid-,  hind-,  and  after-brain  vesicles;  0,  optic  vesicle;  ov,  otic  vesicle;  in,  infundibulum; 
m,  mammillary  body;  pf,  pontine  flexure;  IVv,  fourth  ventricle;  nk,  cervical  flexure;  ol,  olfactory  lobe; 
b,  basilar  artery;   p,  pituitary  recess  (American  Text-Book  of  Obstetrics). 


walls  the  last  eight  cerebral  nerves  take  their  origin.  Its  cavity  forms  the  greater 
part  of  the  fourth  ventricle  which  opens  caudally  into  the  central  canal  of  the 
spinal  cord,  cranially  into  the  cerebral  aqueduct.  The  increase  in  the  flexures  of 
the  brain  and  the  relative  growth  of  its  different  regions  may  be  seen  by  comparing 
the  brains  of  embryos  of  the  third,  fourth,  and  eighth  weeks  (Fig.  315). 

In  the  following  table  are  given  the  primitive  subdivisions  of  the  neural  tube 
and  the  parts  derived  from  them: 


330  THE   MORPHOGENESIS    OF    THE   CENTRAL   NERVOUS    SYSTEM 

THE  DERIVATIVES  OF  THE  NEURAL  TUBE 


Primary  Vesicles                         Subdivisions                                Derivatives 

Cavities 

Telencephalon 

Cerebral  cortex 
Corpora  striata 
Rhinencephalon 

Lateral  ventricles 

Cranial  portion  of 

third  ventricles 

Prosencephalon 

Diencephalon 

Epithalamus 
(pineal  body) 

Thalamus 

Optic  tract 

Hypothalamus 
hypophysis 
tuber  cinereum 
mammillare  bodies 

Third  ventricle 

Mesencephalon 

Mesencephalon 

Corpora  quadrigemina 

Tegmentum 

Crura  cerebri 

Aquaeductus  cerebri 

Rhombencephalon 
Spinal  cord 

Metencephalon 

Cerebellum 
Pons 

Fourth  ventricle 

Myelencephalon 

Medulla  oblongata 

Spinal  cord 

Central  canal 

The  Later  Differentiation  of  the  Subdivisions  of  the  Brain 
Myelencephalon. — We  have  seen  that  .the  wall  of  the  spinal  cord  differen- 
tiates dorsally  and  ventrally  into  roof  plate  and  floor  plate,  laterally  into  the  basal 
plate  and  alar  plate.     The  boundary  line  between  the  basal  and  alar  plates  is  the 


Hoof   plate 
Mantle 


Hoof  plate 


Sulcus  limi 


Ependymal 
layer 


Alar  plate 
S.  limitans 


Basal  plate 


•Spinal 
ganglion 

/';     ■    Ventral  spinal  root 

Fig.  316. — Transverse  sections.     A,  through  the  upper  cervical  region  of  the  spinal  cord  in  a  10  mm. 
human  embryo;   B,  through  the  caudal  end  of  the  myelencephalon  of  the  same.     X  44. 

sulcus  limitans  (Fig.  316  A.).  The  same  subdivisions  may  be  recognized  in  the 
myelencephalon.  It  differs  from  the  spinal  cord,  however,  in  that  the  roof  plate 
is  broad,  thin  and  flattened  to  form  the  ependymal  layer  (Fig.  316  B.).  In  the 
alar  and  basal  plates  of  the  myelencephalon  the  marginal,  mantle  and  ependymal 
zones  are  differentiated  as  in  the  spinal  cord  (Fig.  317  A,  B).     Owing  to  the  for- 


THE    BRAIN 


331 


mation  of  the  pontine  flexure  at  the  beginning  of  the  second  month,  the  roof 
plate  is  broadened,  especially  in  the  cranial  portion  of  the  myelencephalon,  and 
the  alar  plates  bulge  laterally  (Figs.  318  and  319  A).     The  cavity  of  the  myelen- 


Alar  plate 

Sulcus 
Jim  1  tans 

Basal 
plate 

Garry/ion 
juaulare 


'.Hypoqlossus 

\N.  accessor! 'us  N  .vagus    '  MHypog/ossusI 

Fig.  317. — Transverse  sections  through  the  myelencephalon  of  a  10.6  mm.  embryo  (His).  A, 
through  the  nuclei  of  origin  of  the  spinal  accessory  and  hypoglossal  nerves;  B,  through  the  vagus  and 
hypoglossal  nerves  (after  His). 


Inner  layer 


Roof  plaie 


rormaiio  reticularis 
grisea 

Form  at  10  reticularis  alba 


Tractus .  solilarivs 


Rhombic  lip 
Restiform  body 

Z Spinal  V. 

xivr^^ Neuroblasts  from  alar  plate. 
— Marginal  layer 


N.  XII  Septum  medullae        Neuroblasts  from  alar  plate 

r  QRudimenl  or  accessory  olive) 

Fig.  318. — Transverse  section  through  the  myelencephalon  of  an  eight  weeks'  human  embryo  (His) 


cephalon  is  thus  widened  from  side  to  side  and  flattened  dorso-ventrally.  This 
is  most  marked  cranially  where,  between  the  alar  plates  of  the  myelencephalon 
and  metencephalon,  are  formed  the  lateral  recesses  of  the  fourth  ventricle  (Fig. 


332  THE   MORPHOGENESIS    OF   THE   CENTRAL  NERVOUS    SYSTEM 

319  A).  Into  the  ependymal  roof  of  the  myelencephalon  blood-vessels  grow  and, 
invading  the  lateral  recesses,  form  there  the  chorioid  plexus  of  the  fourth  ventricle. 
The  plexus  consists  of  small  finger-like  folds  of  the  ependymal  layer  and  its  cover- 
ing mesenchymal  layer.  The  line  of  attachment  of  the  ependymal  layer  to  the 
alar  plate  is  known  as  the  rhombic  lip  and  later  becomes  the  tcenia  and  obex  of  the 
fourth  ventricle  (Fig.  319  B). 

The  further  growth  of  the  myelencephalon  is  due  (1)  to  the  rapid  formation 
of  neuroblasts,  derived  from  the  ependymal  and  mantle  layers;  (2)  to  the  de- 
velopment of  nerve  fibers  from  these  neuroblasts;  (3)  to  the  development  and 
growth  into  it  of  fibers  from  neuroblasts  in  the  spinal  cord  and  in  other  parts  of 
the  brain. 

The  neuroblasts  of  the  basal  plates  early  give  rise  chiefly  to  the  efferent  fibers 
of  the  cerebral  nerves  (Fig.  317).  They  thus  constitute  motor  nuclei  of  origin 
of  the  trigeminal,  abducens,  facial,  glossopharyngeal,  vagus  complex  and  hypo- 
glossal nerves,  nuclei  corresponding  to  the  ventral  and  lateral  gray  columns  of  the 
spinal  cord.  The  basal  plate  also  produces  part  of  the  reticular  formation  which 
is  derived  in  part  also  from  the  neuroblasts  of  the  alar  plate  (Fig.  318).  The 
axons  partly  cross  as  external  and  internal  arcuate  fibers  and  form  a  portion  of 
the  median  longitudinal  bundle,  a  fasciculus  corresponding  to  the  ventral  ground 
bundles  of  the  spinal  cord.  Other  axons  grow  into  the  marginal  zone  of  the  same 
side  and  form  intersegmental  fiber  tracts.  The  reticular  formation  is  thus  differ- 
entiated into  a  gray  portion  situated  in  the  mantle  zone  and  into  a  white  portion 
located  in  the  marginal  zone  (Fig.  318).  The  marginal  zone  is  further  added  to 
by  the  ascending  fiber  tracts  from  the  spinal  cord  and  the  descending  pyramidal 
tracts  from  the  brain.  As  in  the  cord,  the  marginal  layers  of  each  side  remain 
distinct,  being  separated  by  the  cells  of  the  floor  plate.  The  alar  plates  differ- 
entiate later  than  the  basal  plates.  The  afferent  fibers  of  the  cerebral  nerves 
first  enter  the  mantle  layer  of  the  alar  plates  and  coursing  upward  and  downward 
form  definite  tracts  (tractus  solitarius,  descending  tract  of  fifth  nerve).  To  these 
are  added  tracts  from  the  spinal  cord  so  that  an  inner  gray  and  an  outer  white 
substance  is  formed.  Soon,  however,  the  cells  of  the  mantle  layer  proliferate, 
migrate  into  the  marginal  zone  and  surround  the  tracts.  These  neuroblasts  of  the 
alar  plate  form  groups  of  cells  along  the  terminal  tracts  of  the  afferent  cerebral 
nerves  (which  correspond  to  the  dorsal  root  fibers  of  the  spinal  nerves)  and  con- 
stitute the  receptive  or  terminal  nuclei  of  the  fifth,  seventh,  eighth,  ninth  and  tenth 
cerebral  nerves.  Caudally,  the  nucleus  gracilis  and  nucleus  cuneatus  are  developed 
from  the  alar  plates  as  the  terminal  nuclei  for  the  afferent  fibers  which  ascend  from 


THE    BRAIN 


333 


the  dorsal  funiculi  of  the  spinal  cord.  The  axons  of  the  neuroblasts  forming 
these  receptive  nuclei  decussate  through  the  reticular  formation  chiefly  as 
internal  arcuate  fibers  and  ascend  to  the  thalamus  as  the  median  lemniscus. 

There  are  developed  from  neuroblasts  of  the  alar  plate  other  nuclei  the 
axons  of  which  connect  the  brain  stem,  cerebellum  and  fore-brain.  Of  these  the 
most  conspicuous  is  the  inferior  olivary  nucleus. 

The  characteristic  form  of  the  adult  myelencephalon  is  determined  by  the 
further  growth  of  the  above-mentioned  structures.     The  nuclei  of  origin  of  the 


Mid-brain 


Cerebellum 


Lobule}  of  vermis 


f/occu/i 


■Medulla 
oblongata. 


L  ateral  lobe  of 
Cereb  ellum 


Rhomb, 
lip 


Flocculus 


/\l/n/la. 
Nodulus 


Fig.  319. — Dorsal  views  of  four  stages  in  the  development  of  the  cerebellum.     A,  of  a  13.6  mm.  em- 
bryo (His);  B,  of  a  24  mm.  embryo;  C,  of  a  no  mm.  embryo;  D,  of  a  150  mm.  embryo. 


cerebral  nerves,  derived  from  the  basal  plate,  produce  swellings  in  the  floor  of  the 
fourth  ventricle  which  are  bounded  laterally  by  the  sulcus  limitans.  The  terminal 
nuclei  of  the  mixed  and  sensory  cerebral  nerves  lie  lateral  to  this  sulcus.  The 
enlarged  cuneate  and  gracile  nuclei  bound  the  ventricle  caudally  and  laterally 
as  the  cuneus  and  clava.  The  inferior  olivary  nuclei  produce  lateral  rounded 
prominences  and  ventral  to  these  are  the  large  cerebrospinal  tracts  or  pyramids. 
The  Metencephalon. — Cranial  to  the  lateral  recesses  of  the  fourth  ventricle 
the  cells  of  the  alar  plate  proliferate  ventrally  and  form  the  numerous  and  rela- 
tively large  nuclei  of  the  pons.     The  axons  from  the  cells  of  these  nuclei  mostly 


334 


THE   MORPHOGENESIS    OF   THE   CENTRAL  NERVOUS    SYSTEM 


cross  to  the  opposite  side  and  form  the  brachium  pontis  of  the  cerebellum.  Cere- 
bral fibers  from  the  cerebral  peduncles  end  about  the  cells  of  the  pontine  nuclei. 
Others  pass  through  the  pons  as  fascicles  of  the  pyramidal  tracts. 

Cerebellum. — When  the  alar  plates  of  the  cranial  end  of  the  myelencephalon 
are  bent  out  laterally  the  caudal  portions  of  their  continuations  into  the  meten- 
cephalic  region  are  carried  laterally  also.     As  a  result,  the  alar  plate  of  the  meten- 

cephalon  takes  up  a  transverse 


Mesencephalon 

Cerebellum 

Ependymal 
layer 


B 

Mesencephalon 


Cerebellum 


position  and  forms  the  anlages 
of  the  cerebellum  (Fig.  319  A). 
During  the  second  month  the 
paired  cerebellar  plates  thicken 
and  bulge  into  the  ventricle 
(Fig.  320  A).  Near  the  mid- 
line a  thickening  indicates  the 
anlage  of  the  vermis,  while  the 
remainder  of  the  alar  plates 
form  the  anlages  of  the  lateral 
lobes  or  cerebellar  hemispheres. 

The  cerebellar  anlages  grow 
rapidly  laterally  and  also  in 
length  so  that  their  surfaces  are 
folded  transversely.  During  the 
third  month  their  walls  bulge 
outward  and  form  on  either  side 
a  convex  lateral  lobe  connected 
with  the  pons  by  the  brachium 
pontis  (Fig.  319  C).  In  the 
meantime,  the  anlages  of  the 
vermis  have  fused  in  the  mid- 
line producing  a  single  structure 
marked  by  transverse  fissures.  The  rhombic  lip  gives  rise  to  the  flocculus  and 
nodulus.  Between  the  third  and  fifth  months  the  cortex  cerebelli  grows  more 
rapidly  than  the  deeper  layers  of  the  cerebellum  and  its  principal  lobes,  folds 
and  fissures  are  formed  (Fig.  319  C,  D).  The  hemispheres  derived  from  the 
lateral  lobes  are  the  last  to  be  differentiated.  Their  fissures  do  not  appear  until 
the  fifth  month. 

Cranial  to  the  cerebellum  the  wall  of  the  neural  tube  remains  thin  dorsally 


Post. med.  velum  Ani)med.  velum 

Fig.  320. — Median  sagittal  section  of  the  cerebellum 
and  part  of  mid-brain.  A,  from  a  24  mm.  embryo;  B, 
from  a  150  mm.  embryo.  Ant.  med.  velum,  anterior 
medullary  velum;  Post.  med.  velum,  posterior  medullary 
velum. 


THE    BRAIN 


335 


and  constitutes  the  anterior  medullary  velum  of  the  adult.  Caudally,  the  ependy- 
mal  root"  of  the  fourth  ventricle  becomes  the  posterior  medullary  velum.  The 
points  of  attachment  of  the  vela  remain  approximately  fixed,  while  the  cerebellar 
cortex  grows  enormously.  As  a  result,  the  vela  are  folded  in  under  the  expanding 
cerebellum  (Fig.  320). 

The  anlages  of  the  cerebellum  show  at  first  differentiation  into  the  same  three  layers 
which  are  typical  for  the  neural  tube.  During  the  second  and  third  months  cells  from  the 
ependymal,  and  perhaps  from  the  mantle  layer,  of  the  rhombic  lip,  migrate  to  the  surface  of 
the  cerebellar  cortex  and  give  rise  to  the  molecular  and  granular  layers  which  are  character- 
istic of  the  adult  cerebellar  cortex  (Schafer).  The  later  differentiation  of  the  cortex  is  only 
completed  at  or  after  birth.  The  cells  of  the  granular  layer  become  unipolar  by  a  process  of 
unilateral  growth.  The  Purkinje  cells  differentiate  later.  Their  axons  and  those  of  entering 
afferent  fibers  form  the  deep  medullary  layer  of  the  cerebellum. 


D.  iv 


-*•  Alar  plate 

/ /  ^i!k 

/  /  \k 

A -•Marginal  layer 
-. -Nucleus  N.  Ill 
...  Root  fibers  N.  Ill 


Nu.  IV. 


Fig.  321. — Transverse  sections  through  the  mesencephalon  of  a  10.6  mm.  embryo.  A,  through  the 
isthmus  and  origin  of  the  trochlear  nerve;  B,  through  the  nucleus  of  origin  of  the  oculomotor  nerve 
(His).  D.  IV,  decussation  of  oculomotor  nerve;  MJ.,  mantle  layer;  Nu.  IV.  nucleus  of  oculomotor 
nerve. 


The  cells  of  the  mantle  layer  may  take  little  part  in  the  development  of  the  cerebellar  cor- 
tex, but  give  rise  to  neuroglia  cells  and  fibers  and  to  the  internal  nuclei.  Of  these  the  dentate 
nucleus  may  be  seen  at  the  end  of  the  third  month;  later,  its  cellular  layer  becomes  folded, 
producing  its  characteristic  convolutions.  The  fibers  arising  from  its  cells  form  the  greater 
part  of  the  brack ium  conjunctivum.  (For  a  detailed  account  of  the  development  of  the  cere- 
bellum see  Streeter,  in  Keibel  and  Mall,  vol.  2,  p.  67.) 

Mesencephalon. — The  basal  and  alar  plates  can  be  recognized  in  this  sub- 
division of  the  brain  and  each  differentiates  into  the  three  primitive  layers  (Fig. 
32 1 V  In  the  basal  plate  the  neuroblasts  give  rise  to  the  axons  of  motor  nerves, 
the  oculomotor  cranial,  the  trochlear  caudal  in  position  (Fig.  321  B).  In  ad- 
dition to  these  nuclei  of  origin,  the  nucleus  ruber  (red  nucleus)  is  developed  in 


336  THE   MORPHOGENESIS    OF   THE   CENTRAL   NERVOUS    SYSTEM 

the  basal  plates  ventral  and  somewhat  cranial  to  the  nucleus  of  the  oculomotor 
nerve.  The  origin  of  the  cells  forming  the  red  nucleus  is  not  definitely  known. 
The  alar  plates  form  the  paired  superior  and  inferior  colliculi  which  together 
constitute  the  corpora  quadrigemina  (Fig.  331).  The  plates  thicken  and  neuro- 
blasts migrate  to  their  surfaces,  forming  stratified  ganglionic  layers  comparable 
to  the  cortical  layers  of  the  cerebellum  and  the  cerebellar  nuclei.  With  the 
development  of  the  superior  and  inferior  colliculi  the  cavity  of  the  mesencephalic 
region  decreases  in  size  and  becomes  the  cerebral  aqueduct. 

The  mantle  layer  of  the  basal  plate  region  is  enclosed  ventrally  and  laterally 
by  the  fiber  tracts  which  develop  in  the  marginal  zone.  Ventro-laterally  appear 
the  median  and  lateral  lemnisci  and  ventrally  develop  later  the  descending  tracts 
from  the  cerebral  cortex,  which  together  constitute  the  peduncles  of  the  cerebrum. 


Poof  plate  (with  chorioid  plexus) 


Alar  plate  orThalamus 

Sulcus  limitans  or 
".;  -'' '  I  S.  hypothalamicus 

Basal  plate  or 
Hypothalamus 

' Mammillary  recess 
Fig.  322. — Transverse  section  through  the  diencephalon  of  a  five  weeks'  human  embryo  (His). 

The  Diencephalon. — In  the  wall  of  the  diencephalon  we  may  recognize 
laterally  the  alar  and  basal  plates,  dorsally  the  roof  plate  and  ventrally  the  floor 
plate  (Fig.  322).  The  roof  plate  expands,  folds  as  seen  in  the  figure,  and  into  the 
folds  extend  blood  capillaries.  The  roof  plate  thus  forms  the  ependymal  fining 
of  the  tela  chorioidea  of  the  third  ventricle.  The  vessels  and  ingrowing  mesenchy- 
mal tissue  form  the  chorioid  plexus.  Cranially,  the  tela  chorioidea  roofs  over  the 
median  portion  of  the  telencephalon  and  is  folded  laterally  into  the  hemispheres 
as  the  chorioid  plexus  of  the  lateral  ventricles.  Laterally,  the  roof  plate  is  attached 
to  the  alar  plates  and  at  their  point  of  union  are  developed  the  ganglia  habenulcE. 
The  pineal  body  or  epiphysis  is  developed  caudally  as  an  evagination  of  the  roof 
plate.  Tt  appears  at  the  fifth  week  (Fig.  327)  and  is  well  developed  by  the  third 
month  (Fig.  324).  Into  the  thickened  wall  of  the  anlage  is  incorporated  a  certain 
amount  of  mesenchymal  tissue  and  thus  the  pineal  body  proper  is  formed.     The 


THE    BRAIN 


337 


alar  plate  is  greatly  thickened  and  becomes  the  anlage  of  the  thalamus  and 
metathalamus.  The  latter,  really  a  part  of  the  thalamus,  gives  rise  to  the  lateral 
and  median  geniculate  bodies. 

The  sulcus  liypothalamicus  (Fig.  323)  forms  the  boundary  line  between  the 
thalamus  (alar  plate)  and  the  hypothalamus  (basal  plate  plus  the  floor  plate). 
This  sulcus  thus  corresponds  to  the  sulcus  limitans  of  the  spinal  cord  and 
brain  stem.  The  basal  plate  is  comparatively  unimportant  in  the  dien- 
cephalic  region  as  no  nuclei  of  origin  for  motor  nerves  are  developed  here. 


Sulcus  hypothalami 


Hypothalamus 


Pallium 


Mammillary  recess 
Corpus  striatum         I  Injundibulum 

Optic  ridge 
Fig.  323. — Median  sagittal  section  of  the  fore-  and  mid-brain  regions  of  a  brain  from  a  10.2  mm.  embryo 

(after   His). 


In  the  floor  plate  the  ridge  formed  by  the  optic  chiasma  constitutes  the  pars 
optica  Jiypot/ialamica. 

The  Hypophysis. — The  injundibulum  develops  as  a  recess  caudal  to  the 
pars  optica  hypothalamica  (Figs.  324  and  325).  At  its  extremity  is  the  sac-like 
anlage  of  the  posterior  lobe  of  the  hypophysis  or  pituitary  body.  During  the  fourth 
week  the  infundibular  anlage  comes  into  contact  with  Rathke's  pouch,  the  epi- 
thelial anlage  of  the  anterior  lobe  of  the  hypophysis  (Fig.  325).  The  epithelial 
anlage  is  at  first  flattened  and  soon  is  detached  from  its  epithelial  stalk.  Later, 
it  grows  laterally  and  caudally  about  the  anlage  of  the  posterior  lobe  and  during 
the  second  month  its  wall  is  differentiated  into  convoluted  tubules  which  obliterate 
its  cavity.     The  tubules  become  closed  glandular  follicles  surrounded  by  a  rich 


333 


THE   MORPHOGENESIS    OF   THE   CENTRAL  NERVOUS    SYSTEM 


network  of  blood-vessels  and  produce  an  important  internal  secretion.     Pari 
passu  with  the  differentiation  of  the  anterior  lobe  the  infundibular  anlage  of  the 


Pined  body  epithala, 

Cerebral  peduncle 
Cerebral  aqueduct 

Mesencephalon 
"\    {Mid-brain) 


Spinal  cord 
Central  canal 


Fig.  324. — Median  sagittal  section  of  the  brain  from  a  fetus  of  the  third  month 
(His  from  Sobotta's  Atlas). 


Foramen  Monro 


Third  ventricle 


Optic  vesicle 
Lens  vesicle 


Infundibulum 

HathKe's  pocfat- 

Fig.  325. — Oblique  transverse  section  through  the  diencephalon  and  telencephalon  of  a  10  mm.  embryo. 

X  61. 


posterior  lobe  loses  its  cavity,  but  the  walls  of  the  infundibulum  persist  as  its 
solid  permanent  stalk.     The  lobe  enlarges  and  its  cells  are  differentiated  into  a 


THE   BRAIN 


339 


diffuse  tissue  resembling  neuroglia.  About  the  two  lobes  of  the  hypophysis  the 
surrounding  mesenchyme  develops  a  connective  tissue  capsule. 

Caudal  to  the  infundibulum  in  the  floor  plate  are  developed  in  order  the  tuber 
cinereum  and  the  mammillary  recess  (Figs.  323  and  324).  The  lateral  walls  of 
the  latter  thicken  and  give  rise  to  the  paired  mammillary  bodies. 

The  third  ventricle  lies  largely  in  the  dienccphalon  and  is  at  first  relatively 
broad.  Owing  to  the  thickening  of  its  lateral  walls  it  is  compressed  until  it  forms 
a  narrow  vertical  cleft.  In  a  majority  of  adults  the  thalami  are  approximated, 
fuse  and  form  the  massa  intermedia  or  commissura  mollis,  which  is  encircled  by 
the  cavity  of  the  ventricle. 


Mesencephalon 


Dienccphalon 


Pallium 


Mammillary  body 

Hypophysis 

Optic  stalk  Lobus  olfactorius 

Fig.  326. — Lateral  view  of  the  fore-  and  mid-brains  of  a  10.2  mm.  embryo  (His). 


The  Telencephalon. — This  is  the  most  highly  differentiated  division  of  the 
brain  (Fig.  326).  The  primitive  structures  of  the  neural  tube  can  no  longer  be 
recognized  but  the  telencephalon  is  regarded  as  representing  greatly  expanded 
alar  plates  and  is  therefore  essentially  a  paired  structure.  Each  of  the  paired  out- 
growths expands  cranially,  dorsally,  and  caudally,  and  eventually  overlies  the 
rest  of  the  brain  (Figs.  326,  327  and  328).  The  telencephalon  is  differentiated 
into  the  corpus  striatum,  rhinencephalon,  and  pallium  (primitive  cortex  of  cerebral 
hemisphere).  The  median  lamina  between  the  hemispheres  lags  behind  in  its 
development  and  thus  is  formed  the  great  longitudinal  fissure  between  the  hemi- 
spheres.    The  lamina  is  continuous  caudally  with  the  roof  plate  of  the  dien- 


340 


THE   MORPHOGENESIS    OF   THE   CENTRAL   NERVOUS    SYSTEM 


cephalon,  cranially  it  becomes  the  lamina  terminalis,  the  cranial  boundary  of  the 
third  ventricle. 

Chorioid  Plexus  of  the  Lateral  Ventricles. — It  will  be  remembered  that  in 
the  folds  of  the  roof  plate  of  the  diencephalon  develops  the  chorioid  plexus  of  the 
third  ventricle.  Similarly  the  thin  median  wall  of  the  pallium  at  its  junction 
with  the  wall  of  the  diencephalon  is  folded  into  the  lateral  ventricle.  Into  this 
fold  grows  a  vascular  plexus  continuous  with  that  of  the  third  ventricle  and  pro- 
jects into  the  lateral  ventricle  of  either  side  (Figs.  327  and  329).  The  fold  of  the 
pallial  wall  forms  the  chorioidal  fissure  and  the  vascular  plexus  is  the  chorioid 


Fissura  prima 
Chorioid  plexus  of  lat.  ventricle 

Pallium 


Pineal  body 
Sup.  collicidus 


Corpus  striatum 
Hippocampus 

Roof  plate 


Mesencephalon 


Fig.  327. — The  fore-brain  and  mid-brain  of  an  embryo  13.6  mm.  long  seen  from  the  dorsal  surface. 
The  pallium  of  the  telencephalon  is  cut  away  exposing  the  lateral  ventricle  (His). 

plexus  of  the  lateral  ventricle.  This  is  a  paired  structure  and  with  the  plexus 
of  the  third  ventricle  forms  a  T-shaped  figure,  the  stem  of  the  T  overlying  the 
third  ventricle,  its  curved  arms  projecting  into  the  lateral  ventricles  just  caudal 
to  the  interventricular  foramen.  Later,  as  the  pallium  extends,  the  chorioid 
plexus  of  the  lateral  ventricles  and  the  chorioidal  fissures  are  extensively  elongated 
into  the  temporal  lobe  and  inferior  horn  of  the  lateral  ventricle  (Fig.  330). 

The  interventricular  foramen  (of  Monro)  is  at  first  a  wide  opening  (Fig. 
325)  but  is  later  narrowed  to  a  slit,  not  by  constriction  but  because  its  boundaries 
grow  more  slowly  than  the  rest  of  the  telencephalon  (Fig.  329). 


THE    BRAIN 


341 


The  third  ventricle  extends  some  distance  into  the  caudal  end  of  the  telen- 
cephalon and  laterally  in  this  region  develop  the  optic  vesicles.  Into  each  optic 
stalk  extends  the  optic  recess  (Fig.  325). 


DiencephaAon 


Mesencephalon 


1' all  inn: 


Corpus  mammillarc 


Pars   ant.  olf.  lobe 
Pars  post.  olf.  lobe 


Tuber  cinereutn 

Tnfundibulum        Optic  stalk 
FlG.  328. — Lateral  view  of  the  fore-brain  and  mid-brain  of  a  13.6  mm.  embryo  (His). 


Lateral  ventricle 


Chor 


ioid  plexus  of  lateral j 

ventricle       (,] 

Thalamus  — if 

M 

Corpus  striatum 


Third  ventricle 


FlG.  329. — Transverse  section  through  the  fore-brain  of  a  16  mm.  embryo  showing  the  early  develop- 
ment of  the  chorioid  plexus  and  fissure  (His). 


The  corpus  striatum  is  developed  as  a  thickening  in  the  floor  of  each  cerebral 
hemisphere.  It  is  already  prominent  in  embryos  of  five  weeks  (13.6  mm.)  bulg- 
ing into  the  lateral  ventricle  (Figs.  327  and  329).     It  is  in  line  caudally  with  the 


342 


THE   MORPHOGENESIS    OF    THE   CENTRAL   NERVOUS    SYSTEM 

falx 


Fig.  330. — A  transverse  section  through  the  telencephalon  of  an  83  mm.  embryo  (after  His).  Th, 
thalamus;  cs,  corpus  striatum;  hf,  hippocampal  fissure;  fa,  marginal  gray  seam;  fi,  edge  of  white  sub- 
stance. 


Chorioid  fissure 


Sup.  colliculus 

Inf.  colliculus 
Cerebellum 


Nucleus 
caudatus 

Internal 
capsule 

Olfactory  lobe 


Fig.  331. — Lateral  view  of  the  brain  of  a  53  mm.  fetus.  The  greater  part  of  the  pallium  of  the 
right  cerebral  hemisphere  has  been  removed,  leaving  only  that  covering  the  lenticular  nucleus,  and 
exposing  the  internal  capsule,  caudate  nucleus  and  hippocampus  (His). 


THE    BRAIN 


343 


thalamus  of  the  diencephalon  and  in  development  is  closely  connected  with  it, 
although  the  thalamus  forms  always  a  separate  structure.  The  corpus  striatum 
elongates  as  the  cerebral  hemisphere  lengthens,  its  caudal  portion  curving  around 
to  the  tip  of  the  inferior  horn  of  the  lateral  ventricle  and  forming  the  slender  tail 
of  the  caudate  nucleus  (Fig.  331).  The  thickening  of  the  corpus  striatum  is  due 
to  the  active  proliferation  of  cells  in  the  ependymal  layer  which  form  a  prominent 
mass  of  mantle  layer  cells.  Nerve  fibers  to  and  from  the  thalamus  to  the  cere- 
bral cortex  course  through  the  corpus  striatum  as  laminae  which  are  arranged  in. 


Anterior  horn 


Nucleus  caudatus 

Interventricular 
foramen- 
Third  ventricle 


Chorioid  plexus  of 
lat.  ventricle 


Posterior  horn 


Lenticular  nucleus 
Ant.  columns  of 
fornix 
Internal  capsule 


Thalamus 


Hippocampus 


Fig.  332. — Horizontal  (coronal)  section  through  the  fore-brain  of  a  16  cm.  fetus  (His). 


the  form  of  a  wide  V,  open  laterally,  when  seen  in  horizontal  sections.  This  V- 
shaped  tract  of  white  fibers  is  the  internal  capsule,  the  cranial  limb  of  which  partly 
separates  the  corpus  striatum  into  the  caudate  and  lenticular  nuclei  (Fig.  33  2) .  The 
caudal  limb  of  the  capsule  extends  between  the  lenticular  nucleus  and  the  thalamus. 
The  thalamus  and  corpus  striatum  are  separated  by  a  deep  groove  until  the 
end  of  the  third  month  (Fig.  329).  As  the  structures  enlarge,  the  groove  between 
them  disappears  and  they  form  one  continuous  mass  (Fig.  332).  According  to 
some  investigators,  there  is  direct  fusion  between  the  two. 


344  THE   MORPHOGENESIS    OF   THE   CENTRAL   NERVOUS    SYSTEM 

The  Rhinencephalon  or  Olfactory  Apparatus. — This  is  divided  into  a  basal 
portion  and  a  pallial  portion.  The  basal  portion  consists  (i)  in  a  ventral  and 
cranial  evagination  (pars  anterior)  formed  mesial  to  the  corpus  striatum,  which 
is  the  anlage  of  the  olfactory  lobe  and  stalk  (Fig.  328).  This  receives  the  olfactory 
fibers  and  its  cells  give  rise  to  olfactory  tracts.  The  tubular  stalk  connecting  the 
olfactory  lobe  with  the  cerebrum  loses  its  lumen.  (2)  Caudal  to  the  anlage  of  the 
olfactory  lobe  a  thickening  of  the  brain  wall  develops  (pars  posterior)  which  ex- 
tends mesially  along  the  lamina  terminalis  and  laterally  becomes  continuous 
with  the  tip  of  the  temporal  lobe  (Figs.  323  and  328).  This  thickening  consti- 
tutes the  anterior  perforated  space  and  the  parolfactory  area  of  the  adult  brain. 

The  pallial  portion  of  the  rhinencephalon  is  termed  the  archipallium  because 
it  forms  the  primitive  wall  of  the  cerebrum.  It  forms  a  median  strip  of  the  pallial 
wall  curving  along  the  dorsal  edge  of  the  chorioidal  fissure  from  the  anterior 
perforated  space  around  to  the  tip  of  the  temporal  lobe,  where  it  is  again  con- 
nected with  the  basal  portion  of  the  rhinencephalon.  The  archipallium  differ- 
entiates into  the  hippocampus,  a  portion  of  the  gyrus  hippocampi  and  into  the 
gyrus  dentatus.  It  resembles  the  rest  of  the  cerebral  cortex  in  the  arrangement 
of  its  cells.     The  infolding  of  the  hippocampus  produces  the  hippocampal  fissure. 

The  Commissures  of  the  Telencephalon. — The  important  commissures  are 
the  corpus  callosum,  fornix  and  anterior  commissure.  The  first  is  the  great  trans- 
verse commissure  of  the  neopallium  or  cerebral  cortex,  while  the  fornix  and  an- 
terior commissure  are  connected  with  the  archipallium  of  the  rhinencephalon. 
The  commissures  develop  in  relation  to  the  lamina  terminalis,  crossing  partly  in 
its  wall  and  partly  in  fused  adjacent  portions  of  the  median  pallial  walls.  Owing 
to  the  fusion  of  the  pallial  walls  dorsal  and  cranial  to  it,  the  lamina  terminalis 
thickens  rapidly  in  stages  between  80  and  150  mm.  (Streeterin  Keibeland  Mall, 
vol.  2).  "It  [the  lamina  terminalis]  is  distended  dorsal  ward  and  antero-lateral- 
ward  through  the  growth  of  the  corpus  callosum,  the  shape  of  which  is  determined 
by  the  expanding  pallium."  Between  the  curve  of  the  corpus  callosum  and  the 
fornix  a  space  is  formed,  the  fifth  ventricle,  or  space  of  the  septum  pellucidum  (Fig. 
333  A,  B).  This  space  is  bounded  laterally  by  a  portion  of  the  median  pallial 
wall  which  remains  thin  and  membranous,  and  constitutes  the  septum  pellucidum 
of  the  adult. 

The  fornix  takes  its  origin  early,  chiefly  from  cells  in  the  hippocampus. 
The  fibers  course  along  the  chorioidal  side  of  the  hippocampus  cranially,  pass- 
ing dorsal  to  the  foramen  of  Monro  (Fig.  333  A).  In  the  cranial  portion  of  the 
lamina  terminalis  fibers  are  given  off  and  received  from  the  basal  portion  of  the 


THE   BRAIN 


345 


rhinencephalon.  In  this  region  libers  crossing  the  midline  form  the  hippocampal 
commissure.  Other  libers,  as  the  anterior  pillars  of  the  fornix,  curve  ventrally 
and  end  in  the  mammillary  body  of  the  hypothalamus.  The  commissure  of  the 
hippocampus,  originally  cranial  in  position,  is  carried  caudal  ward  with  the 
caudal  extension  of  the  corpus  callosum  (Fig.  333  B). 


Corpus  callosum 


Hippocampal  commissure 
Anterior  commissure 

Arit.  pillars  of  form* 


Body  oF  fornix 


Chorioid  fissure 
Thalamus 


Body  of  fornix 
Jeptum  pellucidum 


Hippocampal  commissure 

Corpus  callosum 


Ant.  commissure ' 

■Thalamus 

Yirit. pillar  of  fornix 

Fig.  333. — Two  stages  in  the  development  of  the  cerebral  commissure.  A,  Median  view  of  the 
right  hemisphere  of  an  83  mm.  embryo;  B,  the  same  of  a  120  mm.  embryo.  (Based  on  reconstructions 
by  His  and  Streeter). 


The  fibers  of  the  anterior  commissure  cross  in  the  lamina  terminalis  ventral 
to  the  hippocampal  commissure.  They  arise  as  a  cranial  and  a  caudal  division. 
The  fibers  of  the  former  take  their  origin  from  the  olfactory  stalk  and  the  adjacent 
cortex.  The  fibers  of  the  caudal  division  pass  ventrally  about  the  corpus  striatum 
between  it  and  the  cortex,  and  may  be  derived  from  one  or  both  of  these  regions. 

The  corpus  callosum  appears  cranial  and  dorsal  to  the  hippocampal  com- 


346 


THE  MORPHOGENESIS  OF  THE  CENTRAL  NERVOUS  SYSTEM 


missure  in  the  roof  of  the  thickened  lamina  terminalis  (Fig.  333  A).  Its  fibers 
arise  from  neuroblasts  in  the  wall  of  the  neopallium  (cerebral  cortex)  and  by  them 
nearly  all  regions  of  one  hemisphere  are  associated  with  corresponding  regions 
of  the  other.  With  the  expansion  of  the  pallium  the  corpus  callosum  is  extended 
cranially  and  caudally  by  the  development  of  interstitial  fibers.  The  fibers  first 
found  in  the  corpus  callosum  arise  in  the  median  wall  of  the  hemispheres.  In 


Lateral 
fissure 


Lobus 
frontalis 


Lobus 

temporalis 

Pons 


Lobus  parietalis 


£ 

T.nhiis 
occipitalis 

Cerebellum 

w  ■  ■' 

Myelenceph 

. 

alon 

Spinal  cord 


Fig.  334. — Lateral  view  of  the  brain  of  a  90  mm.  embryo  (His). 


fetuses  of  150  mm.  (five  months)  this  great  commissure  is  a  conspicuous  structure 
and  shows  the  form  which  is  characteristic  of  the  adult  (Fig.  333  B). 

The  Form  of  the  Cerebral  Hemispheres. — When  the  telencephalon  expands 
cranially,  caudally  and  at  the  same  time  ventrally.  four  lobes  may  be  distin- 
guished (1)  a  cranial  frontal  lobe;  (2)  a  dorsal  parietal  lobe;  (3)  a  caudal  occipi- 
tal lobe,  and  (4)  a  ventro-lateral  temporal  lobe  (Fig.  334).  The  ventricle  extends 
into  these  regions  and  in  each  forms  respectively  the  anterior  horn,  the  body,  the 
posterior  horn  and  the  inferior  horn  of  the  lateral  ventricle.     The  surface  extent 


THE    HRAIN 


347 


■of  the  cerebral  wall,  the  thin  gray  cortex,  increases  more  rapidly  than  the  un- 
derlying white  medullary  layer.  As  a  result  the  cortex  is  folded,  producing  i  on- 
volutions  between  which  are  depressions,  the  sulci  and  fissures.    The  i  borioidal 

fissure  is  formed,  as  we  have  seen,  by  the  ingrowth  of  the  chorioid  plexus. 
During  the  third  month  the  hippocampal  fissure  develops  as  a  curved  infolding 
along  the  median  wall  of  the  temporal  lobe.  The  infolded  cortex  forms  the 
hippocampus.  The  lateral  fissure  (of  Sylvius)  makes  its  appearance  also  in  the 
third  month,  but  its  development  is  not  completed  until  after  birth.  The  cortex 
overlying  the  corpus  striatum  laterally  develops  more  slowly  than  the  surrounding 


Occipital  lobe  of 
cerebrum 


Corpora 
quadrigemina 

Impression  of 
thalamus 

Hemisphere  of \___^EL-  ^\  I  I ^ Temporal  lobe 

cerebellum 

Lateral  recess  of 
Vermis  cerebelli  — — ""  ^'^^  V  j^f^F" ventricle  4 

Fasciculus  gracilis 
Medulla  oblongata 

J' u;-  335- — Posterior  view  of  the  brain  from  a  100  mm.  embryo  (Kollmann's  Handatlas). 

areas  and  is  thus  gradually  overgrown  by  folds  of  the  parietal  and  frontal  lobes 
(fronto-parietal  operculum)  and  of  the  temporal  lobe  (temporal  operculum). 
The  area  thus  overgrown  is  the  i>isula  (island  of  Reil)  and  the  depression  so  formed 
is  the  lateral  fissure  (of  Sylvius).  Later,  frontal  and  orbital  opercula  are  developed 
ventro-laterally  from  the  frontal  lobe  (Fig.  337).  These  are  not  approximated 
over  the  insula  until  after  birth.  The  frontal  operculum  is  included  between  the 
anterior  limbs  of  the  Sylvian  fissure  and  the  extent  of  its  development,  which  is 
variable,  determines  the  form  of  these  limbs. 

In  fetuses  of  six  to  seven  months  four  other  depressions  appear  which  later 
form  important  landmarks  in  the  cerebral  topography.     These  are:    (1)   the 


548 


THE   MORPHOGENESIS    OF    THE   CENTRAL   NERVOUS    SYSTEM 


central  sulcus,  or  fissure  of  Rolando,  which  forms  the  dorso-lateral  boundary  fine 
between  the  frontal  and  parietal  lobes  (Fig.  337);  (2)  the  parieto-occipital  fissure, 
which,  on  the  median  wall  of  the  cerebrum,  is  the  fine  of  separation  between  the 


Median  olfactory 
gyrus 

Middle  olfactory 

gyrus 

Diagonal  gyrus 


Cerebellum 


Insula 

Lat.  olfactory  gyrus 

Gyrus  ambiens 
Gyrus  semilunaris 


Oliva 


Fig.  336. — Ventral  view  of  the  brain  of  a  100  mm.  embryo  showing  development  of  the  rhinencephalon 

(Kollmann). 


Sulcus  postcentralis 


Sulcus  centralis 


Lobus 

parielalis 

superior 

Supra- 
marginal  \ 
and  an- 
gular gyri 

Post. 

ramus 

of  lateral 

fissure 


Middle 

temporal 

sulcus 

Occipital 
pole 


Inferior 

frontal 

sulcus 

Ascend- 


Temporal 
lobe 


Superior  temporal  gyrus         Middle  temporal  gyrus 
FlG.  337.-4-Lateral  view  of  the  right  cerebral  hemisphere,  from  a  seven  months'  fetus  (Kollmann). 


THE    BRAIN 


349 


occipital  and  parietal  lobes  (Fig.  338);  (3)  the  odcarinc  fissure  which  includes 
between  it  and  the  parieto-occipital  fissure  the  cuneus  and  marks  the  position  oi 

the  visual  area  of  the  cerebrum;  (4)  the  collateral  fissure  on  the  ventral  surface 
of  the  temporal  lobe,  which  produces  the  inward  bulging  on  the  floor  of  the 
posterior  horn  of  the  ventricle  known  as  the  collateral  eminence.  The  calcarine 
fissure  also  affects  the  internal  wall  of  the  ventricle,  causing  the  convexity 
termed  the  calcar  avis. 

Simultaneously  with  the  development  of  the  collateral  fissure  appear  other 
shallower  depressions  known  as  sulci.     These  have  a  definite  arrangement  and 


Sparc  of 

septum 

pellu- 

cidum 

Rostral 

lamina 

Parol- 
factory 
area 


Corp.  callosum 
singuli 


Side.  corp.  callosi 
Splenium 


ial  fissure 


Cuneus 


Olfactory  lobe  Fissura  rliinica 

Optic  nerve     Temporal  lobe 

Fig.  338. — Median  surface  of  the  right  cerebral  hemisphere  from  a  seven  months'  fetus  (Kollmann). 


with  the  fissures  mark  off  from  each  other  the  various  functional  areas  of  the  cere- 
brum. The  surface  convolutions  between  the  depressions  constitute  the  gyri 
and  lobules  of  the  adult  cerebrum. 

Histogenesis  of  the  Cerebral  Cortex. — The  three  primitive  zones  typical 
of  the  neural  tube  are  differentiated  in  the  wall  of  the  pallium:  the  ependymal, 
mantle  and  marginal  layers.  During  the  first  two  months  the  cortex  remains 
thin  and  differentiation  is  slow.  At  eight  weeks  neuroblasts  migrate  from  the 
ependymal  and  mantle  zones  into  the  marginal  zone  and  give  rise  to  layers  of 
pyramidal  cells  typical  of  the  cerebrum.  The  differentiation  of  these  layers  is 
most  active  during  the  third  and  fourth  months.     From  the  fourth  month  on  the 


350  THE    MORPHOGENESIS    OF    THE    CENTRAL   NERVOUS    SYSTEM 

cerebral  wall  thickens  rapidly  owing  to  the  development  of  (i)  the  fibers  from  the 
thalamus  and  corpus  striatum;  (2)  of  endogenous  fibers  from  the  neuroblasts  of 
the  cortex.  The  fibers  form  a  white  inner  medullary  layer  surrounded  by  the  gray 
cortex.  As  the  cerebral  wall  increases  in  thickness  the  size  of  the  lateral  ventricle 
becomes  relatively  less,  its  lateral  diameter  especially  being  decreased.  For  the 
special  differentiation  of  the  cerebral  cortex  in  different  regions  the  student  is- 
referred  to  text-books  on  neurology. 


CHAPTER  XII 

THE  PERIPHERAL  NERVOUS  SYSTEM 

The  nerves,  ganglia  and  sense  organs  constitute  the  peripheral  nervous 
system.  The  peripheral  nerves  consist  of  bundles  of  medullated  and  non-medul- 
lated  nerve  libers  and  aggregations  of  nerve  cells,  the  ganglia.  The  fibers  are  of 
two  types:  Afferent  fibers  which  carry  sensory  impulses  to  the  central  nervous 
system,  and  efferent  fibers,  which  carry  effective  impulses  away  from  the  nervous 
centers.  The  peripheral  efferent  fibers  of  both  brain  and  spinal  cord  take  their 
origin  from  neuroblasts  of  the  basal  plate.  Typically  they  emerge  ventro-later- 
ally  from  the  neural  tube.  Those  arising  from  the  spinal  cord  take  origin  in  the 
mantle  layer,  converge  and  form  the  ventral  roots  of  the  spinal  neroes.  The 
efferent  fibers  of  the  brain  take  origin  from  more  definite  nuclei  and  constitute 
the  motor  or  effector  portions  of  the  cerebral  nerves.  The  peripheral  afferent  fibers 
take  origin  from  nerve  cells  which  lie  outside  the  neural  tube.  Those  sensory 
nerve  cells  related  to  the  spinal 
cord  and  to  the  brain  stem  caudal 
to  the  otic  vesicle  are  derived  from 
the  ganglion  crest,  the  origin  of 
which  has  been  described  (Chapter 
X,p.  314). 


K-Xigang.  crest. 


xi  fibres. 


bridge. 


Fig.  339. — Reconstruction  of  an  embryo  of  4  mm. 
showing  the  development  of  the  cerebrospinal  nerves. 
Ci.,  2.,  etc.,  cervical  spinal  nerves  (Streeter). 


A.    SPINAL  NERVES 

The  spinal  nerves  are  segment- 
ally  arranged  and  each  consists  of 
dorsal    and    ventral   roots,  spinal 

ganglion  and  nerve  trunks.  In  embryos  of  4  mm.  the  ventral  roots  are  already 
developing  as  outgrowths  of  neuroblasts  in  the  mantle  layer  of  the  spinal  cord 
(Fig.  339).  The  spinal  ganglia  are  represented  as  enlargements  along  the  ganglion 
crest  and  are  connected  by  a  bridge  of  cells. 

In  7  mm.  embryos  (four  weeks  old)  the  cells  of  the  spinal  ganglia  begin  to 
develop  centrally  directed  processes  which  enter  the  marginal  zone  of  the  cord  as 
the  dorsal  root  fibers  (Fig.  340).     These  fibers  course  in,  and  eventually  form  the 

351 


352 


THE   PERIPHERAL   NERVOUS    SYSTEM 


greater  part  of,  the  dorsal  funiculi.  Peripheral  processes  of  the  ganglion  cells 
join  the  ventral  root  fibers  in  the  trunk  of  the  nerve  (Fig.  342).  At  10  mm.  (Fig. 
341)  the  dorsal  root  fibers  have  elongated  and  the  cellular  bridges  of  the  ganglion 
crest  between  the  spinal  ganglion  have  begun  to  disappear.  In  transverse  sec- 
tions at  this  stage  (Figs.  307  and  342)  the  different  parts  of  a  spinal  nerve  may  be 
seen.     The  trunk  of  the  nerve  just  ventral  to  the  union  of  the  dorsal  and  ventral 


Ophthal.  dio. 
Sup.  max.  div, 
N.masticatorius 
Inf.  max. div. 


xi  gang,  crest. 


Fig.  340. — Reconstruction  of  a  6.9  mm.  embryo  showing  the  development  of  the  dorsal  root  fibers  from 
the  spinal  and  cerebral  ganglia  (Streeter).     X  16.7. 


roots  gives  off  laterally  the  dorsal,  or  posterior  ramus,  the  fibers  of  which  supply 
the  dorsal  muscles.  The  ventral  ramus  continuing  gives  off  mesially  the  ramus 
communicans  to  the  sympathetic  ganglion  and  divides  into  the  lateral  and  ventral 
(anterior)  terminal  rami.  The  efferent  fibers  of  these  rami  supply  the  muscles 
of  the  lateral  and  ventral  body  wall  and  the  afferent  fibers  end  in  the  integument 
of  the  same  regions. 


SPINAL    NERVES 


353 


At  the  points  where  the  anterior  and  lateral  terminal  rami  arise,  connecting 
loops  may  extend  from  one  spinal  nerve  to  another.  Thus  in  the  cervical  region 
superficial  and  deep  nerve  plexuses  are  formed.  The  deep  cervical  plexus  forms 
the  ansa  hypoglossi  and  the  phrenic  nerve. 


Vesicula  auditiva 
Gang,  acusticum 


Cans,  semilunare  n.V 
Cerebellum 


Gang,  radicls  n  IX 
:Gang.  petrosuni 


Gang,  radicls  n.X 


N.  frontalis   " 


Gang.  Frorlep 
N.  hypoglossus 

I.C 

"  -*-N.XI. 

~  Gang,  nodos. 

— N.  desc.  cerv. 
--■Kami  hyoid. 

(Ansa  hypoglossi) 
-— N.  Musculocutan. 
;N.  axillaris 

N.  phrenicus 
■~N.  medianus 
--  N.  radialis 
™N.  ulnaris 

ITh- 


I  Co 
N.  tibialis 
N.  peroneus 

Tubus  digest. 


N.  femoral    j 
N.  obturator 


R.  posterior 

It.  terminalis  lateralis 
R.  terminalis  anterior 
Mesonephros 


Nn.  ilioing.  et  hypogaatr. 
Fig.  341. — Reconstruction  of  the  nervous  system  of  a  10  mm.  embryo  (Streeter).     X  12. 


The  Brachial  and  Lumbo-sacral  Plexuses.- -The  nerves  supplying  the  arm 
and  leg  also  unite  to  form  plexuses.  In  embryos  of  10  mm.  (Fig.  341)  the  trunks 
of  the  last  four  cervical  nerves  and  of  the  first  thoracic  are  united  to  form  a  flat- 
tened plate,  the  anlage  of  the  bracliial  plexus.     From  this  plate  nervous  cords 

23 


354 


THE    PERIPHERAL   NERVOUS    SYSTEM 


extend  into  the  intermuscular  spaces  and  end  in  the  premuscle  masses.  The 
developing  skeleton  of  the  shoulder  splits  the  brachial  plexus  into  dorsal  and 
ventral  laminae  from  which  the  various  nerves  to  the  arm  and  shoulder  arise. 

In  10  mm.  embryos  the  lumbar  and  sacral  nerves  which  supply  the  leg  unite 
in  a  plate-like  structure,  the  anlage  of  the  lumbosacral  plexus  (Fig.  341).  The 
plate  is  divided  by  the  skeletal  elements  of  the  pelvis  and  femur  into  two  lateral 
and  two  median  trunks.  Of  the  cranial  pair  the  lateral  becomes  the  femoral 
nerve;  the  median,  the  obturator  nerve.  The  caudal  pair  constitute  the  sciatic 
nerve;   the  lateral  trunk  is  the  peroneal  nerve,  and  the  median  trunk  is  the  tibial. 

Save  for  the  neurones  from  the  special  sense  organs  (nose,  eye  and  ear)  which 


Dorsal  not 


Somah 
neurone 

Visceral  sensory 
neurone. 


Marginal  layer 

Ependymal  laye. 
Mantle  layer 


Lot. 

Term-. 
a 1 vi 5/  on 


Ventral  terminal 
division  of  spinal  nerve 

Ramus  commumcans 


Sympathetic  ganglion 


Fig.  342. — Transverse  section  of  a  10  mm.  embryo  showing  the  spinal  cord,  spinal  nerves  and  their 
function  nervous  components.     Diagrammatic. 


form  a  special  sensory  group,  the  neurones  of  the  peripheral  nerves,  both  spinal 
and  cerebral,  fall  into  four  function  groups  (Fig.  342). 

(1)  Somatic  afferent,  or  general  sensory,  with  fibers  ending  in  the  integument 
of  the  body  wall. 

(2)  Visceral  afferent,  or  sensory,  with  fibers  ending  in  the  walls  of  the  viscera. 

(3)  Somatic  efferent,  or  motor,  with  fibers  ending  on  voluntary  muscle 
fibers. 

(4)  Visceral  efferent,  or  motor,  fa)  with  fibers  ending  about  sympathetic 
ganglion  cells,  which  in  turn  control  the  smooth  muscle  fibers  of  the  viscera  and 
blood-vessels  fspinal  nerves) ;  or  fb)  with  fibers  ending  directly  on  visceral  muscle 


THE    CEREBRAL    NERVES  355 

fibers  (mixed  cerebral  nerves).     The  relation  of  the  sympathetic  system  to  the 
central  nervous  system  is  described  on  page  364. 


B.     THE  CEREBRAL  NERVES 
The  cerebral  nerves  of  the  human  brain  are  twelve  in  number.     They  differ 
from  the  spinal  nerves:    (1)  in  that  they  are  not  segmentally  arranged,  and  (2) 
in  that  they  do  not  all  contain  the  same  types  of  nervous  components.     Classed 
according  to  the  functions  of  their  neurones  they  fall  into  three  groups. 


Special  Somatic 

Somatic  Motor 

Visceral  Sensory 

Sknsory 

or  Effector 

and  Motor 

I.  Olfactory. 

III.  Oculomotor. 

V. 

Trigeminal. 

II.  Optic. 

IV.  Trochlear. 

VII. 

Facial. 

VIII.  Acoustic. 

VI.  Abducens. 

IX. 

Glossopharyngeal. 

XII.  Hypoglossal. 

X. 

Vagus  complex  including 

XI. 

Spinal  Accessory. 

It  will  be  seen  (1)  that  the  nerves  of  the  first  group  are  purely  sensory, 
corresponding  to  the  general  somatic  afferent  neurones  of  the  spinal  nerves;  (2) 
that  the  nerves  of  the  somatic  motor  group  are  purely  motor  and  correspond  to  the 
somatic  efferent  or  motor  neurones  of  the  spinal  nerves;  (3)  that  the  nerves  of  the 
third  group  are  of  mixed  function  and  correspond  to  the  visceral  components  of 
the  spinal  nerves. 

I.    The  Special  Somatic  Sensory  Nerves 

1.  The  Olfactory  Nerve  though  purely  sensory  has  no  ganglion.  Its  nerve 
cells  lie  at  first  in  the  olfactory  epithelium  of  the  nose  and  are  of  the  bipolar  type. 
From  these  cells  peripheral  processes  develop  and  end  directly  at  the  surface  of  the 
olfactory  epithelium.  Central  processes  grow  into  contact  with  the  olfactory 
lobe  and  form  the  strands  of  the  olfactory  nerve.  They  end  in  the  olfactory 
bulb  about  the  peculiar  mitral  cells.  Some  of  the  olfactory  cells  migrate  from 
epithelium  along  with  the  developing  nerve  fibers,  and  may  be  found  as  bipolar 
cells  along  the  course  of  the  nerve.  The  olfactory  nerve  fibers  are  peculiar  in 
that  they  remain  non-medullated. 

When  the  ethmoidal  bone  of  the  cranium  is  developed  its  cartilage  is  formed 
around  the  strands  of  the  olfactory  nerve,  which  thus  in  the  adult  penetrate  the 
cribriform  plate  of  the  ethmoid. 

Nerve  fibers  which  pass  from  the  epithelium  of  the  organ  of  Jacobson  also 
end  in  the  olfactory  bulb.  The  organ  of  Jacobson  is  a  vestigial  sense  organ  and 
its  nerve  is  rudimentary.     For  the  development  of  the  olfactory  organ  see  p.  369. 


356  THE   PERIPHERAL   NERVOUS    SYSTEM 

2.  The  Optic  Nerve  is  formed  by  fibers  which  take  their  origin  from  neuro- 
blasts in  the  nervous  layer  of  the  retina.  The  retina  is  differentiated  from  the 
wall  of  the  fore-brain  and  remains  attached  to  it  by  the  optic  stalk  (Fig.  325). 
The  neuroblasts  from  which  the  optic  nerve  fibers  develop  constitute  the  gang- 
lion cell  layer  of  the  retina  (Fig.  362).  During  the  sixth  and  seventh  weeks  these 
cells  give  rise  to  central  processes  which  form  a  nerve  fiber  layer  on  the  inner  side 
of  the  retina.  The  optic  fibers  converge  to  the  optic  stalk  and  grow  through  its 
wall  back  to  the  brain.  The  cells  of  the  optic  stalk  are  converted  into  a  neu- 
roglia framework  and  the  cavity  .is  obliterated.  In  the  floor  of  the  fore-brain 
at  a  point  which  forms  the  boundary  line  between  telencephalon  and  dienceph- 
alon,  the  fibers  from  the  median  half  of  each  retina  cross  to  the  opposite  side 
(decussate),  and  this  crossing  constitutes  the  optic  chiasma  (from  Greek  letter  X  or 
"chi").  The  decussation  of  the  optic  fibers  takes  place  about  the  end  of  the  sec- 
ond month.  The  crossed  and  uncrossed  fibers  constitute  the  optic  tract  which 
rounds  the  cerebral  peduncles  laterally  and  dorsally  (Fig.  336).  Eventually,  the 
optic  fibers  end  in  the  lateral  geniculate  body,  thalamus  and  superior  colliculus. 

8.  The  Auditory  Nerve,  or  N.  Acusticus,  is  formed  by  fibers  which  originate 
from  the  cells  of  the  acoustic  ganglion.  The  origin  of  these  cells  is  unknown, 
though  they  appear  in  4  mm.  embryos  just  cranial  to  the  otic  vesicle  (Fig.  340). 
The  cells  become  bipolar,  central  processes  uniting  the  ganglion  to  the  tuberculum 
acusticum  of  the  myelencephalon  and  peripheral  fibers  connecting  it  with  the 
wall  of  the  otocyst.  The  acoustic  ganglion  is  differentiated  into  the  vestibular  and 
spiral  ganglia  (Fig.  343).  Its  development  has  been  studied  by  Streeter  (Amer. 
Jour.  Anat.,  vol.  6).  The  ganglion  elongates  and  is  subdivided  into  superior 
and  inferior  portions  in  7  mm.  embryos.  The  superior  part  supplies  nerves  to 
the  utriculus  and  to  the  anterior  and  lateral  semicircular  canals.  It  forms  part 
of  the  vestibular  ganglion  of  the  adult.  The  inferior  portion  supplies  nerves  to  the 
sacculus  and  to  the  ampulla  of  the  posterior  semicircular  canal  and  this  portion 
of  it  with  the  pars  superior  constitutes  the  vestibular  ganglion.  The  greater 
part  of  the  pars  inferior  is,  however,  differentiated  into  the  spiral  ganglion,  the 
peripheral  fibers  of  which  innervate  the  hair  cells  of  the  spiral  organ  (of  Corti) 
in  the  cochlea.  The  spiral  ganglion  appears  in  9  mm.  embryos  and  conforms  to 
the  spiral  turns  of  the  cochlea,  hence  its  name.  Its  central  nerve  fibers  form  the 
cochlear  division  of  the  acoustic  nerve.  This  is  distinctly  separated  from  the 
central  fibers  of  the  vestibular  ganglion  which  constitute  the  vestibular  division 
of  the  acoustic  nerve,  the  fibers  of  which  are  not  auditory  in  function.  The  pars 
inferior  of  the  vestibular  ganglion  becomes  closely  connected  with  the  n.  coch- 


THE  CEREBRAL  NERVES 


357 


-n.vestib.--- 


4mm.        /mm.  re 


-pars  sup. 


— pars  inf: 


pars  sup.  ,pars  sup. 

nar^.'nf  ^         ,X^rn-v<?stib---(V       J  -pars  inf? 

pars  inr.,^  y,  v 


. 


-9  mm.- 


"-/ 


"■  n.  coch. 
"''*&;«$•- qang.  spirale  ■ 


r.  amp.  sup.v 
r.  amp.  lat.-~^ 


n.coch  — 


^M\ n.vestib. "/^^l 


3  K        ---^ 

r.  utri  c. <Xg*mmJnfli 


la 


■  20mm 


*%•:.•:.-.•  •■.■•■.•■•..•.v-~ 


qanq.       / 
spirale  ^--. 


^"' 


p.  amp.  sup. 


<V* 


•<5^ 


MEDIAN  VIEW 


LATERAL  VIEW 


Fig.  343. — The  development  of  the  acoustic  ganglia  and  nerves.     The  vestibular  ganglion  is  finely 
stippled,  the  spiral  ganglion  coarsely  stippled      (Streeter  in  Amer.  Jour.  Anat.). 


358  THE    PERIPHERAL   NERVOUS    SYSTEM 

leans,  and  thus  in  the  adult  it  appears  as  though  the  sacculus  and  posterior 
ampulla  were  supplied  by  the  cochlear  nerve.  The  cells  of  both  the  vestibular 
and  spiral  ganglia  are  of  the  bipolar  type. 

n.    The  Somatic  Motor  Nerves 

The  nerves  of  this  group  consisting  of  the  three  nerves  to  the  eye  muscles 
and  the  n.  hypoglossus  are  purely  motor  nerves,  the  fibers  of  which  take  origin 
from  the  neuroblasts  of  the  basal  plate  of  the  brain  stem,  near  the  midline.  They 
are  regarded  as  the  homologues  of  the  ventral  motor  roots  of  the  spinal  cord,  but 
have  lost  their  segmental  arrangement  and  are  otherwise  modified.  The  nuclei 
of  origin  of  these  nerves  are  shown  in  Fig.  345. 

12.  N.  Hypoglossus. — This  nerve  is  formed  by  the  fusion  of  the  ventral  root 
fibers  of  three  to  five  precervical  nerves.  Its  fibers  take  origin  from  neuroblasts 
of  the  basal  plate  and  emerge  from  the  ventral  wall  of  the  myelencephalon  in 
several  groups  (Fig.  339).  In  embryos  of  the  fourth  week  (7  mm.)  the  fibers 
have  converged  ventrally  to  form  the  trunk  of  the  nerve  (Fig.  340) .  Later  they 
grow  cranially  lateral  to  the  ganglion  nodosum  and  eventually  end  in  the  muscle 
fibers  of  the  tongue  (Fig.  341).  The  nerve  in  its  development  unites  with  the 
cervical  nerves  to  form  the  ansa  hypoglossi.  Its  nucleus  of  origin  is  shown  in 
Fig.  345- 

That  the  hypoglossal  is  a  composite  nerve  homologous  with  the  ventral  roots  of  the 
spinal  nerves  is  shown:  (1)  by  the  segmental  origin  of  its  fibers;  (2)  from  the  fact  that  its  nucleus 
of  origin  is  a  cranial  continuation  of  the  ventral  gray  column,  or  nucleus  of  origin  for  the  ventral 
spinal  roots;  (3)  from  the  fact  that  in  mammalian  embryos  (pig,  sheep,  cat,  etc.)  rudimentary 
dorsal  ganglia  are  developed,  one  of  which  at  least  (Froriep's  ganglion)  sends  a  dorsal  root  to  the 
hypoglossal.  In  human  embryos  Froriep's  ganglion  may  be  present  as  a  rudimentary  structure 
(Figs.  341  and  344),  or  it  may  be  absent  and  the  ganglion  of  the  first  cervical  nerve  may  also 
degenerate  and  disappear.  In  pig  embryos  the  writer  has  found  two  and  three  accessory 
ganglia  (including  Froriep's)  from  which  dorsal  roots  extended  to  the  root  fascicles  of  the  hypo- 
glossal nerve  (Fig.  116). 

3.  The  Oculomotor  Nerve,  as  we  have  seen,  takes  origin  from  neuroblasts 
in  the  basal  plate  of  the  mesencephalon  (Tig.  321  B).  The  fibers  emerge  as 
small  fascicles  on  the  ventral  surface  of  the  mid-brain  in  the  concavity  due  to  the 
cephalic  flexure  (Figs.  341  and  345).  The  fascicles  converge,  form  the  trunk  of 
the  nerve  and  end  in  the  premuscle  masses  of  the  eye.  The  nerve  eventually 
supplies  all  of  the  extrinsic  muscles  of  the  eye  save  the  superior  oblique  and  ex- 
ternal rectus.  A  branch  is  also  supplied  to  the  ciliary  ganglion.  In  the  chick 
embryo  bipolar  cells  migrate  along  the  fibers  of  the  oculomotor  nerve  to  take  part 


THE   CEREBRAL   NERVES  359 

in  the  development  of  the  ganglion.     The  ciliary  ganglion  of  human  embryos  is 
derived  entirely  from  the  semilunar  ganglion  of  the  trigeminal  nerve. 

4.  The  Trochlear  Nerve  fibers  take  origin  from  neuroblasts  of  the  basal 
plate,  located  just  caudal  to  the  nucleus  of  origin  of  the  oculomotor  nerve.  They 
are  directed  dorsally,  curve  around  the  cerebral  aqueduct  and,  crossing  in  its 
roof,  emerge  at  the  isthmus  (Fig.  321  A).  From  their  superficial  origin  they 
are  directed  ventrally  as  a  slender  nerve  which  connects  with  the  anlage  of  the 
superior  oblique  muscle  of  the  eye  (Fig.  341). 

6.  The  N.  Abducens  takes  origin  from  a  nucleus  of  cells  in  the  basal  plate  of 
the  myelencephalon  located  directly  beneath  the  fourth  metamere  of  the  floor  of 
the  fourth  ventricle  (Figs.  341  and  345).  The  converging  fibers  emerge  ventrally 
at  a  point  caudal  to  the  future  pons,  and  as  a  single  trunk  course  cranially  mesial 
to  the  semilunar  ganglion,  finally  ending  in  the  anlage  of  the  external  rectus 
muscle  of  the  eye.  Vestigial  rootlets  of  the  abducens  and  hypoglossal  nerve 
tend  to  fill  in  the  gap  between  these  two  nerves,  according  to  Bremer  and  Elze. 

III.  The  Visceral  Mixed  Nerves 
The  nerves  of  this  group,  the  trigeminal,  facial,  glossopharyngeal  and  vagus 
complex  (vagus  plus  the  spinal  accessory)  are  mixed  in  function.     The  trigem- 
inal nerve,  beside  its  visceral  nerve  components,  contains  also  numerous  somatic 
sensory  neurones  which  supply  the  integument  of  the  head  and  face. 

5.  The  Trigeminal  Nerve  is  largely  sensory.  Its  semilunar  ganglion  is  the 
largest  of  the  whole  nervous  system  and  is  a  derivative  of  the  ganglion  crest,  but 
very  early  is  distinct  from  the  other  cerebral  ganglia  (Fig.  340).  It  arises 
laterally  at  the  extreme  cranial  end  of  the  hind-brain.  Central  processes  from 
its  cells  form  the  large  sensory  root  of  the  nerve  which  enters  the  wall  of  the  hind- 
brain  at  the  level  of  the  pontine  flexure  (Fig.  341).  The  peripheral  processes 
separate  into  three  large  divisions,  the  ophthalmic,  maxillary  and  mandibular 
rami,  and  supply  the  integument  of  the  head  and  face  and  the  epithelium  of  the 
mouth  and  tongue.  The  central  fibers  fork  and  course  cranially  and  caudally 
in  the  alar  plate  of  the  myelencephalon.  The  caudal  fibers  constitute  the  des- 
cending spinal  tract  of  the  trigeminal  nerve,  which  extends  as  far  caudalward  as 
the  spinal  cord  (Fig.  345). 

The  motor  fibers  of  the  trigeminal  nerve  arise  chiefly  from  a  dorsal  motor 
nucleus  which  lies  opposite  the  point  at  which  the  sensory  fibers  enter  the  brain 
wall  (Fig.  345).  In  the  embryo  these  fibers  emerge  as  a  separate  motor  root, 
course  along  the  mesial  side  of  the  semilunar  ganglion,  and  as  a  distinct  trunk 


36° 


THE    PERIPHERAL   NERVOUS    SYSTEM 


supply  the  premuscle  masses  which  later  form  the  muscles  of  mastication.  From 
the  chief  motor  nucleus  a  line  of  cells  extending  cranially  into  the  mesencephalon 
constitutes  a  second  source  of  origin  for  motor  fibers.  In  the  adult,  the  motor 
fibers  form  a  part  of  the  mandibular  division  of  the  nerve. 

The  facial,  glossopharyngeal  and  vagus  nerves  are  essentially  visceral  in  func- 
tion. Their  sensory  fibers,  chiefly  of  the  visceral  type,  supply  the  sense  organs 
of  the  visceral  arches  and  viscera.  These  fibers  originate  in  the  ganglia  of  their 
respective  nerves  and,  entering  the  alar  plate  of  the  myelencephalon,  course 
caudally  as  the  solitary  tract  (Fig.  345).     A  few  somatic  sensory  fibers  having 

Vagus  root  gang,  {jugular). 

Accessory  root  gang. 


Sup.  gang, 
n.  IX 


Gang.petro 

Gang.nodos, 
N.  laiyng.  sup. 


Gang. 
Froriep. 


Rad.  dors. 


Inter- gang, 
bridge.. 


Fig.  344. — Reconstruction  of  the  cerebrospinal  nerves  of  an  embryo  of  10.2  mm.  (Streeter).     X  16.7. 


the  same  origin  and  course  in  the  myelencephalon  supply  the  adjacent  integu- 
ment. 

7.  The  Facial  Nerve  is  largely  composed  of  efferent  motor  fibers  which  supply 
the  facial  muscles  of  expression  (Fig.  341).  In  10  mm.  embryos  these  fibers  arise 
from  a  cluster  of  neuroblasts  in  the  basal  plate  of  the  myelencephalon  located 
beneath  the  third  rhombic  groove  or  neuromere  (Fig.  345).  The  fibers  from  these 
cells  course  laterally,  and  emerge  just  mesial  to  the  acoustic  ganglion.  The  motor 
trunk  then  courses  caudally  and  is  lost  in  the  tissue  of  the  hyoid  visceral  arch, 
tissue  which  later  gives  rise  to  the  muscles  of  expression.     The  sensory  fibers  of 


THE   CEREBRAL   NERVES 


361 


the  facial  nerve  arise  from  the  cells  of  the  geniculate  ganglion,  which  are  in  turn 
derived  from  the  ganglion  crest  (Streeter).  This  ganglion  is  present  at  the  third 
week  (Fig.  341),  located  cranial  to  the  acoustic  ganglion.  The  centrally  directed 
processes  of  the  geniculate  ganglion  enter  the  alar  plate  and  form  part  of  the 
solitary  tract.  The  peripheral  fibers  in  part  course  with  motor  fibers  in  the  chorda 
tympani,  join  the  mandibular  branch  of  the  trigeminal  nerve  and  end  in  the  sense 
organs  of  the  tongue.  Other  sensory  fibers  form  later  the  great  superficial  petrosal 
nerve,  which  extends  to  the  spheno-palatine  ganglion. 

The  motor  fibers  of  the  facialis  at  first  course  straight  laterally  passing 


Nucl.  motor.  n.X  (amblguus) 


Tractus  solltarlus 


Radix  sens.  n.X 
Rr.  mot.  n.X 


G»ng.  radlcls  n.X  Tractus  spinalis  n.V 


1  Radix  sens.  n.lX 


Nucl.  motor,  n.  trigemlnl 

N  trocblearli- 
Gang.  semilunars 


Gang,  genlculatum 
1  Radix  sens.  n.VII 
I    t  para  lutertned.) 


Tr.  cerebcll.  n.V. 


Nucl.  n.  hjrpoglossl 


/ 

'           ^_^B       N. cerv  1 

H 

posterior 

1     Gang,  nodos. 

,                 1     ~-     R.  mot.  ve 

ntr.     1 

■T 

R.  mot.  lat. 

R.  hyold 

R.  posterior 

N 

vagus 

Nucl. 
mot. 
n  IX 


N.  facialis 
N.  bypoglossus 


1  N.  abducens 
N.  maxillaris 


Portlo  minor 
N.  inandlbularls 


N  frontalis 
-N.  nasoclllniis 


Fig.  345. — Reconstruction  of  the  nuclei  of  origin  and  termination  of  the  cerebral  nerves  in  an  embryo 
of   10  mm.     The  somatic  motor  nuclei  are  colored  red  (Streeter).     X  30. 


cranial  to  the  nucleus  of  the  abducens.  The  nuclei  of  the  two  nerves  later 
gradually  shift  their  positions,  that  of  the  facial  nerve  moving  caudally  and 
lateralwards,  while  the  nucleus  of  the  abducens  shifts  cranialwards.  As  a  result, 
the  motor  root  of  the  facial  nerve  in  the  adult  bends  around  the  nucleus  of  the 
abducens  producing  the  genu  or  knee  of  the  former.  The  two  together  produce 
the  rounded  eminence  in  the  floor  of  the  fourth  ventricle  known  as  the  facial  collic- 
ulus. 

9.  The  Glossopharyngeal  Nerve  takes  its  superficial  origin  just  caudal  to  the 
otic  vesicle  (Figs.  340,  344  and  346).  Its  few  motor  fibers  arise  from  neuroblasts 
in  the  basal  plate  beneath  the  fifth  neuromeric  groove.     These  neuroblasts  form 


362  THE    PERIPHERAL   NERVOUS    SYSTEM 

part  of  the  nucleus  ambiguus,  a  nucleus  of  origin  which  the  glossopharyngeal 
shares  with  the  vagus  (Fig.  345).  The  motor  fibers  course  laterally  beneath  the 
spinal  tract  of  the  trigeminal  nerve  and  emerge  to  form  the  trunk  of  the  nerve. 
These  libers  later  supply  the  muscles  of  the  pharynx. 

The  sensory  fibers  of  the  glossopharyngeal  nerve  arise  from  two  ganglia,  a 
superior  or  root  ganglion  and  a  petrosal  or  trunk  ganglion  (Figs.  341  and  346). 
These  fibers  constitute  the  greater  part  of  the  nerve  and  peripherally  divide  to 
form  the  tympanic  and  lingual  rami.  Centrally,  these  fibers  enter  the  alar 
plate  of  the  myelencephalon  and  join  the  sensory  fibers  of  the  facial  nerve 
coursing  caudally  in  the  solitary  tract. 

10.  n.  The  Vagus  and  Spinal  Accessory. — The  vagus,  like  the  hypoglossal, 
is  composite,  representing  the  union  of  several  nerves  which  in  aquatic  animals 
supply  the  branchial  arches  (Figs.  341  and  346).  The  more  caudal  fascicles  of 
motor  fibers  take  their  origin  in  the  lateral  gray  column  of  the  cervical  cord  as  far 
back  as  the  fourth  cervical  segment.  The  fibers  emerge  laterally  and,  as  the 
spinal  accessory  trunk  (in  anatomy  a  distinct  nerve),  course  cranialwards  along 
the  line  of  the  neural  crest  (Figs.  340,  341  and  346).  Other  motor  fibers  take  their 
origin  from  the  neuroblasts  of  the  nucleus  ambiguus  of  the  myelencephalon  (Fig. 
345) .  Still  others  arise  from  a  dorsal  motor  nucleus  which  lies  median  in  position. 
The  fibers  from  these  two  sources  emerge  laterally  as  separate  fascicles  and  join 
the  fibers  of  the  spinal  accessory  in  the  trunk  of  the  vagus  nerve.  The  accessory 
fibers  soon  leave  the  trunk  of  the  vagus  and  are  distributed  laterally  and  caudally 
to  the  visceral  premuscle  masses  which  later  form  the  sterno-cleido-mastoideus  and 
trapezius  muscles  of  the  shoulder  (Fig.  341).  Other  motor  fibers  of  the  vagus 
supply  muscle  fibers  of  the  pharynx  and  larynx. 

As  the  vagus  is  a  composite  nerve  it  has  several  root  ganglia  which  arise  as 
enlargements  along  the  course  of  the  ganglion  crest  (Figs.  341  and  346).  The 
more  cranial  of  these  ganglia  is  the  ganglion  jugular e.  The  others  are  termed 
accessory  ganglia,  are  vestigial  structures  and  not  segmentally  arranged.  In 
addition  to  the  root  ganglia  of  the  vagus  the  ganglion  nodosum  forms  a  ganglion 
of  the  trunk  (Fig.  346).  The  trunk  ganglia  of  both  the  vagus  and  glossopharyn- 
geal nerves  are  believed  to  be  derivatives  of  the  ganglion  crest,  their  cells  migrating 
ventrally  in  early  stages. 

The  central  processes  from  the  neuroblasts  of  the  vagus  ganglia  enter  the 
wall  of  the  myelencephalon,  turn  caudalward,  and  with  the  sensory  fibers  of  the 
facial  and  glossopharyngeal  nerves  complete  the  formation  of  the  solitary  tract. 


THE    CEREBRAL    NERVES 


36.S 


The  peripheral  processes  of  the  ganglion  cells  form  the  greater  part  of  the  vagus 
trunks  after  the  separation  from  it  of  the  spinal  accessory  fibers. 


Vagus  root  gang. 

Accessory  root  gang. 


ix  root  g 


Gang,  petro 
N.  tymp. 


Br  to 
carotid  pit 


N.  laryq.  sup. 


Gang,  nodo s. 


Sympathetic 
Fig.  346. — A  reconstruction  of  the  peripheral  nerves  in  an  embryo  of  17.5  mm.  (Streeter).     X  16.7. 


In  aquatic  vertebrates  special  somatic  sensory  fibers  from  the  lateral  line  organs  join  the 
facial,  glossopharyngeal  and  vagus  nerves,  and  their  ganglion  cells  form  part  of  the  geniculate, 
petrosal  and  nodose  ganglia.  In  human  embryos  the  organs  of  the  lateral  line  are  represented 
by  ectodermal  thickenings  or  placodes  which  occur  temporarily  over  the  geniculate,  petrosal 


364  THE   PERIPHERAL   NERVOUS    SYSTEM 

and  nodose  ganglia.    The  nervous  elements  supplying  these  vestigial  organs  have  completely 
disappeared. 


C.    THE  SYMPATHETIC  NERVOUS  SYSTEM 
The  sympathetic  nervous  system  is  composed  of  a  series  of  ganglia  and 
peripheral  nerves,  the  fibers  of  which  supply  gland  cells  and  the  smooth  muscle 
fibers  of  the  viscera  and  blood-vessels.     It  may  function  independently  of  the 
central  nervous  system  and  is  hence  known  as  the  autonomic  system. 

The  sympathetic  ganglion  cells  are  derived  from  the  cells  of  the  ganglion 
crest.  At  an  early  stage  (6  to  7  mm.)  certain  cells  of  this  crest  migrate  ventrally 
and  give  rise  to  a  series  of  ganglia  which,  in  the  region  of  the  trunk,  are  segmentally 
arranged  (Fig.  342).  The  migration  of  the  sympathetic  cells  is  rapidly  taking 
place  in  embryos  of  6  to  7  mm.  At  9  mm.  the  ganglionated  cord  is  formed  and 
fibers  connecting  the  sympathetic  ganglia  with  the  spinal  nerves  constitute  the 
rami  communicantes  (Streeter).  The  more  peripheral  ganglia  (cardiac  and 
cceliac)  and  the  sympathetic  ganglia  of  the  head  may  be  found  in  16  mm.  embryos 

(Fig.  347)- 

The  cells  which  are  to  form  the  ganglia  of  the  sympathetic  chain  migrate 
ventrally  ahead  of  the  efferent  fibers  and  take  up  a  position  lateral  to  the  aorta. 
The  ganglionic  anlages  are  at  first  distinct  but  unite  with  each  other  from  segment 
to  segment,  forming  a  longitudinal  cord  of  cells.  After  the  formation  of  the  rami 
communicantes  by  the  root  fibers  from  the  spinal  nerves  centripetal  processes 
from  the  sympathetic  cells  grow  back  and  join  the  trunks  of  the  spinal  nerves. 
The  visceral  spinal  fibers  later  become  medullated  and  constitute  the  white  rami; 
the  sympathetic  centripetal  fibers  remain  non-medullated  and  form  the  gray  rami 
of  each  ramus  communicans  (Fig.  342).  From  neurones  of  the  ganglionated 
cord  nerve  fibers  extend  from  ganglion  to  ganglion  and  thus  the  cellular  cord  is  in 
part  converted  into  a  fibrous  longitudinal  commissure.  This  commissure  con- 
nects the  persisting  cellular  masses  which  constitute  the  sympathetic  ganglia  of 
each  segment.  In  the  head  region  the  sympathetic  ganglia  are  not  segmentally 
arranged  but  they  are  derived  from  cells  of  the  cerebrospinal  ganglia  which 
migrate  to  a  ventral  position  (Fig.  346).  These  cells  likewise  give  rise  to  nerve 
fibers  which  constitute  longitudinal  commissures  connecting  the  various  ganglia 
of  the  head  with  the  ganglionated  cord  of  the  trunk  region.  The  small  cranial 
sympathetic  ganglia  are  probably  all  derived  from  the  anlage  of  the  semilunar 
ganglion  (Fig.  347).  The  ciliary  ganglion  is  related  by  a  ramus  communicans  to 
the  ophthalmic  division  of  the  trigeminal  nerve  and  receives  fibers  from  the 


THE    SYMPATHETIC    NKRVOUS    SYSTEM 


365 


oculomotor  nerve.     Its  cells  are  probably  derived  entirely  from  the  semilunar 
ganglion. 

The  sphenopalatine  and  submaxillary  ganglia  probably  take  their  origin 
from  migrating  cells  of  the 
semilunar  ganglion,  but  as 
they  are  connected  with 
the  geniculate  ganglion  of 
the  facial  nerve  some  of 
their  cells  may  be  derived 
from  this  ganglion.  The 
sphenopalatine  ganglion  is 
connected  directly  with 
the  semilunar  ganglion  by 
two  communicating  rami. 
The  submaxillary  ganglion 
is  intimately  related  to  the 
mandibular  division  of  the 
trigeminal  and  through  it 
with  the  semilunar  gang- 
lion, while  the  otic  ganglion 
is  united  to  it  by  a  plexus 
and  is  related  to  the  glosso- 
pharyngeal nerve  through 
its  tympanic  branch. 

The  cervical  ganglia 
lose  their  segmental  ar- 
rangement and  represent 
the  fusion  of  from  two  to 
five  chain  ganglia  of  the 
cervical  and  upper  thoracic 
region.  The  more  distally 
located  prevertebral  and  vis- 
ceral ganglia  are  derived  from  cells  of  the  neural  crest  which  migrate  to  a  greater 
distance  ventrally  (Fig.  347).  The  cardiac  and  cosliac  plexuses  may  be  seen  in 
16  mm.  embryos. 

The  sympathetic  nerve  cells  give  rise  to  axons  and  dendrites  and  are  thus 


Fig.  347. — The  sympathetic  system  in  a  16  mm.  human 
embryo  (after  Streeter).  The  ganglionated  trunk  is  heavily 
shaded.  The  first  and  last  cervical,  thoracic,  lumbar,  sacral  and 
coccygeal  spinal  ganglia  are  numbered,  a,  aorta;  arc,  accessory 
nerve;  car,  carotid  artery;  cil,  ciliary  ganglion;  coe,  cceliac 
artery;  ///,  heart;  nod,  nodose  ganglion;  ot,  otic  ganglion;  pet, 
petrosal  ganglion;  s-m,  submaxillary  ganglion;  s.  mes.,  superior 
mesenteric  artery:  sph.-p.,  sphenopalatine  ganglion;  spl,  splanch- 
nic nerve;    st.,  stomach.  (Lewis-Stohr.) 


366 


THE   PERIPHERAL   NERVOUS    SYSTEM 


topically  multipolar  cells.    Their  axons  possess  a  neurilemma  sheath  but  remain 
non-medullated. 


V\ 


m,z 


■-jgato&L- 


sy 
J. 


D.  CHROMAFFIN  BODIES:  SUPRARENAL  GLAND 
Certain  cells  of  the  sympathetic  ganglia  do  not  form  nerve  cells  but  are 
transformed  into  peculiar  gland  cells  which  produce  an  internal  secretion.     The 
secretion  formed  by  these  cells  causes  them  to  stain  brown  when  treated  with 

chrome  salts,  hence  they  are  called 
chromaffin  cells.  Cells  of  this  type 
derived  from  the  ganglionated  cord 
of  the  sympathetic  system  give  rise 
to  structures  known  as  chromaffin 
bodies.  Chromaffin  derivatives  of 
the  cceliac  plexus,  together  with  mes- 
enchymal tissue,  form  the  anlage  of 
the  suprarenal  gland,  an  organ  which 
reaches  a  relatively  large  size  in  hu- 
man embryos. 

The  Chromaffin  Bodies  of  the 
ganglionated  cord  are  rounded  cellu- 
lar masses  partly  embedded  in  the 
dorsal  surfaces  of  the  ganglia  (Fig. 
348).  At  birth  they  may  attain  a 
diameter  of  1  to  1.5  mm.  In  num- 
ber they  vary  from  one  to  several 
for  each  ganglion. 

Similar  chromaffin  bodies  may 
occur  in  all  the  larger  sympathetic 
plexuses.  One  of  these,  associated  with  the  intercarotid  plexus,  is  the  carotid  gland, 
which  is  thus  re  ^arded  as  a  derivative  of  sympathetic  chromaffin  tissue.  The  anlage 
has  been  first  observed  in  20  mm.  embryos.  The  largest  of  these  structures  found 
in  the  abdominal  sympathetic  plexuses  are  the  aortic  chromaffin  bodies.  These 
occur  on  either  side  of  the  inferior  mesenteric  artery  ventral  to  the  aorta  and 
mesial  to  the  metanephros.  At  birth  they  attain  a  length  of  9  to  12  mm.  and 
are  composed  of  cords  of  chromaffin  cells  intermingled  with  strands  of  connective 
tissue,  the  whole  being  surrounded  by  a  connective  tissue  capsule.  After  birth 
the  chromaffin  bodies  degenerate  but  do  not  disappear  entirely. 


'oCL, 


-■'9.  ?,'•'■■  '}  %  /m 
■■■■  '■  / 


Fig.  348. — Section  through  a  chromaffin  body 
in  a  44  mm.  human  embryo  (after  Kohn).  p,  p, 
mother  chromaffin  cells;  sy,  sympathetic  cells;  b, 
blood-vessel. 


CHROMAFFIN    BODIES;    SUPRARENAL   GLAND 


367 


The  Suprarenal  Gland  is  developed  from  chromaffin  tissue  which  becomes 
its  medulla,  and  from  mesodermal  tissue  which  gives  rise  to  its  cortex.  In  an 
embryo  of  6  mm.  the  anlage  of  the  cortex  is  present,  according  to  Soulie,  and  is 
derived  from  a  thickening  of  the  coelomic  epithelium.  At  8  mm.  the  glands  are 
definite  organs  and  at  9  mm.  their  vascular  structure  is  evident.  The  cellular 
elements  of  the  cortex  are  at  first  larger  than  the  chromaffin  cells  which  give  rise 


V'V*V" 


cap- 


» *  * 


"   . 


FlG.  349. — Transverse  section  through  right  suprarenal  gland  of  a  15.5  mm.  embryo  (after  T.  H. 
Bryce).  sy,  sympathetic  cells;  sy',  groups  of  cells  extending  from  the  sympathetic  into  the  suprarenal 
gland;  cap,  capsule  of  gland;  a,  aorta. 


to  the  medulla.     The  anlages  of  the  glands  form  projections  in  the  dorsal  wall  of 
the  ccelom  between  the  mesonephros  and  mesentery  (Figs.  214  and  225). 

The  chromaffin  cells  of  the  medulla  are  derived  from  the  cceliac  plexus  of  the 
sympathetic  system.  In  embryos  of  15  to  19  mm.  (Fig.  349)  masses  of  these 
cells  begin  to  migrate  from  the  median  side  of  the  suprarenal  anlage  to  a  central 
position,  and  later  surround  the  central  vein  which  is  present  in  embryos  of  23 
mm.  The  primitive  chromaffin  cells  are  small  and  stain  intensely.  They  con- 
tinue their  immigration  until  after  birth.     The  differentiation  of  the  cortex  into 


368  THE    PERIPHERAL    NERVOUS    SYSTEM 

its  three  characteristic  layers  is  not  completed  until  between  the  second  and  third 
years.  The  inner  reticular  zone  is  formed  first,  next  the  fasciculate  zone  and  last 
the  glomerular  zone. 

When  the  cells  of  the  medulla  begin  to  produce  an  internal  secretion  they 
give  the  chrome  reaction.  By  using  extract  of  the  aortic  bodies,  which  are  entirely 
composed  of  chromaffin  cells,  Biedl  and  Wiesel  have  proved  that  its  effect,  like 
that  of  adrenalin,  is  to  increase  the  blood  pressure.  The  logical  conclusion  is 
that  the  effect  of  adrenalin,  an  extract  of  the  suprarenal  glands,  is  due  to  an 
internal  secretion  produced  by  the  chromaffin  cells  of  the  suprarenal  medulla. 

Portions  of  the  suprarenal  anlage  may  be  separated  from  the  parent  gland  and  form 
accessory  suprarenals.  As  a  rule,  such  accessory  glands  are  composed  only  of  cortical  substance 
and  may  migrate  some  distance  from  their  original  position,  accompanying  the  genital  glands. 


E.  DEVELOPMENT  OF  THE  SENSE  ORGANS 
The  nervous  structures  of  the  sense  organs  are  derived  from  the  ectoderm 
and  consist  of  the  general  sense  organs  of  the  integument,  muscles,  tendons  and 
viscera,  and  of  the  special  sense  organs  which  include  the  taste  buds  of  the  tongue, 
the  olfactory  epithelium,  the  retina,  optic  nerve  and  lens  of  the  eye,  and  the 
epithelial  lining  of  the  ear  labyrinth. 

I.  General  Sensory  Organs 

Free  nerve  terminations  form  the  great  majority  of  all  the  general  sensory 
organs.  When  no  sensory  corpuscle  is  developed,  the  neurofibrils  of  the  sensory 
nerve  fibers  separate  and  end  among  the  cells  of  the  epidermis. 

Lamellated  corpuscles  first  arise  during  the  fourth  and  fifth  months  as  masses 
of  mesodermal  cells  clustered  around  a  nerve  termination.  These  cells  increase 
in  number,  flatten  out  and  give  rise  to  the  concentric  lamellae  of  these  peculiar 
structures.     In  the  cat  these  corpuscles  increase  in  number  by  budding. 

The  tactile  corpuscles,  according  to  Ranvier,  are  developed  from  mesenchymal 
cells  and  branching  nerve  fibrils  during  the  first  six  months  after  birth. 

II.  Taste  Buds 

The  sense  of  taste  resides  chiefly  in  the  taste  buds  of  the  tongue.  The  de- 
velopment of  the  tongue  has  been  described  (p.  158)  and  we  may  speak  here  only 
of  the  development  and  distribution  of  the  taste  buds. 

In  the  fetus  of  five  to  seven  months  taste  buds  are  more  widely  distributed 
than  in  the  adult.     They  are  found  in  the  walls  of  the  vallate,  fungiform  and  foliate 


DEVELOPMENT  OF  THE  SENSE  ORGANS 


369 


papillae  of  the  tongue,  on  the  under  surface  of  the  tongue,  on  both  surfaces  of  the 
epiglottis,  on  the  palatine  tonsils  and  arches  and  on  the  soft  palate.  After  birth 
many  of  the  taste  buds  degenerate,  persisting  on  the  lateral  walls  of  the  vallate 
and  foliate  papillae,  on  a  few  fungiform  papilla*  and  on  the  laryngeal  surface  of  the 
epiglottis. 

The  anlages  of  the  taste  buds  appear  as  thickenings  of  the  lingual  epithelium 
in  11  cm.  fetuses  (Keibel).  The  cells  of  the  taste  bud  anlage  lengthen  and  later 
extend  to  the  surface  of  the  epithelium.  They  are  differentiated  into  the  sensory 
taste  cells  with  modified  cuticular  tips  and  into  supporting  cells.  The  taste  buds 
are  suppled  by  nerve  fibers  of  the  seventh,  ninth  and  tenth  cerebral  nerves; 
the  fibers  branch  and  end  in  contact  with  the  walls  of  the  taste  cells. 


B 


Olfactory  plate 

D 


Median  nasal 
process 


Fore- or 


■  Organ  of 
Jacob  son 


Telencephalon 
rJasal  fossa. 

Lat nasal  process 
-Med.  -nasal  process 
Maxillary    process 


Nasal  fossa  epithelial   plate 

Fig.  350. — Sections  through  the  olfactory  anlages  of  human  embryos.     A,  4.9  mm.;  5,6.5  mm.;  C,  8.8 
mm.;   D  and  E.  10  mm.  embryo.     (.4,  B  and  C  from  Keibel  and  Elze.) 


ELL    The  Olfactory  Organ 
The  olfactory  epithelium  arises  as  paired  thickenings  or  placodes  of  the 
cranial  ectoderm  (Fig.  350  A).     The  placodes  are  bent  inward  to  form  the  nasal 
fossce  about  which  the  nose  develops. 

In  embryos  of  4  to  5  mm.  the  placodes  are  sharply  marked  off  from  the  sur- 
24 


sr 


THE    PERIPHERAL    NERVOUS    SYSTEM 


rounding  ectoderm  as  ventro-lateral  thickenings  near  the  tip  of  the  head.  They 
are  flattened  and  begin  to  invaginate  in  embryos  of  6  to  7  mm.  In  8  mm.  embryos 
the  invagination  has  produced  a  distinct  pit  or  fossa  surrounded  everywhere  save 
ventrally  by  a  marginal  swelling. 

The  later  development  of  the  olfactory  organ  is  associated  with  that  of  the 
face.  It  will  be  remembered  (see  p.  153)  that  the  first  branchial  arch  forks  into 
the  maxillary  and  mandibular  processes.  Dorsal  to  the  oral  cavity  is  the  frontal 
process  of  the  head,  lateral  to  it  the  maxillary  processes,  and  ventral  to  it  are  the 
mandibular  processes  (Fig.  144).     With  the  development  of  the  nasal  pits  the 


Nasal  septum 


Ext.naris 

lot.  nasal 
process 

Med.  nasal 
process 

Maxillary 
process 

/Vfa/idible 


ExT.naris 
Lat.  nasal  process 


Oral  cavity 


Maxillary 
process 

Mandible 

Med.  nasal  process 


Oral  cavity 


Fig.  351. — Two  stages  in  the  development  of  the  jaws  and  nose.     A ,  ventral  view  of  the  end  of  the  head 
of  a  10.5  mm.  embryo  (after  Peter);  B,  of  an  11.3  mm.  embryo  (after  Rabl). 


frontal  process  is  divided  into  paired  lateral  nasal  processes  and  a  single  median 
frontal  process,  from  which  later  are  differentiated  the  median  nasal  processes,  or 
processus  globular es  (Fig.  351).  The  nasal  pits  are  at  first  grooves,  each  bounded 
mesially  by  the  median  frontal  process  and  laterally  by  the  lateral  nasal  process 
and  the  maxillary  process  (Fig.  351  A).  The  fusion  of  the  maxillary  processes  with 
the  ventro-lateral  ends  of  the  median  frontal  process  converts  the  nasal  grooves 
into  blind  pits  or  fossae,  shutting  them  off  from  the  mouth  cavity  (Fig.  351  A,  B). 
Thus  in  embryos  of  10  to  12  mm.  the  nasal  fossa  has  but  one  opening,  the  external 
naris,  and  is  separated  from  the  mouth  cavity  by  an  ectodermal  plate  (Fig.  350 
D,  E). 


DEVELOPMENT  OF  THE  SENSE  ORGANS 


371 


The  ventro-lateral  ends  of  the  median  frontal  process  enlarge  and  become 
the  median  nasal  processes  which  fuse  with  the  lateral  nasal  processes  and  re- 
duce the  size  of  the  external  nares  (Fig.  351  B).  Externally,  the  nares  are  now 
bounded  ventrally  by  the  fused  nasal  processes.  The  epithelial  plates  whi<  b 
separate  the  nasal  fossae  from  the  primitive  mouth  cavity  become  thin  membra- 
nous structures  caudally  and,  rupturing,  produce  two  internal  nasal  openings,  the 
primitive  choance  (Fig.  148).  Cranially,  the  epithelial  plate  is  destroyed  by  in- 
growing mesoderm  of  the  maxillary  process  and  median  nasal  process  which 


Olfactory 
epithelium 


Organ  of 
Jacobs o 


Inferior 
concha 


Palatine 
process 


Dental 
lamina 


Cartilage  of 
nasal  septum 


Cartilage  of 
Jacobson's  organ 


Naso-lachry. 
nial  duct 


Ongue 


-MecKel's 
carh/age 


Fig.  352.- — Transverse  section  through  the  nasal  passages  and  palatine  processes  of  a  20  mm.  embryo. 
In  the  nasal  septum  is  seen  a  section  of  the  organ  of  Jacobson.     X  30. 


replaces  it  and  constitutes  the  primitive  palate  (Fig.  350  D).  The  primitive  palate 
forms  the  lip  and  the  premaxillary  palate.  The  nasal  fossae  now  open  externally 
through  the  external  nares  and  internally  into  the  roof  of  the  mouth  cavity 
through  the  primitive  choanal. 

Coincident  with  these  changes  the  median  frontal  process  has  become  rela- 
tively smaller  and  that  portion  of  it  between  the  external  nares  and  the  nasal 
fossae  becomes  the  nasal  septum  (Fig.  351  A,  B).  As  the  facial  region  grows  and 
elongates,  the  primitive  choanae  become  longer  and  form  slit-like  openings  in  the 
roof  of  the  mouth  cavity.     By  the  development  and  fusion  of  the  palatine  pro- 


372 


THE   PERIPHERAL   NERVOUS    SYSTEM 


cesses  (described  on  p.  156)  the  dorsal  portion  of  the  mouth  cavity  is  separated 
off  and  constitutes  the  nasal  passages  (compare  Figs.  352  and  353).  The  nasal 
passages  of  the  two  sides  for  a  time  communicate  through  the  space  between  the 
hard  palate  and  the  nasal  septum.  Later,  the  ventral  border  of  the  septum  fuses 
with  the  hard  palate  and  completely  separates  the  nasal  passages.  The  nasal 
passages  of  the  adult  thus  consist  of  the  primitive  nasal  fossae  plus  a  portion  of  the 
primitive  mouth  cavity  which  has  been  separated  off  secondarily  by  the  develop- 
ment of  the  hard  palate.  The  passages  of  the  adult  thus  open  caudally  by  sec- 
ondary choanae  into  the  cavity  of  the  pharynx. 

The  epithelium  which  lines  the  nasal  fossae  is,  a  portion  of  it,  transformed  into 


Olfactory  epithelium 

Ethmo-turbinal  J 
Nasal  septum 


Naso-lachry- 
maL  duct 


-  Mail  II 0- 
turbinal 


Fig.  353. — Transverse  section  through  the  nasal  passages  of  a  65  mm.  embryo.     X  14- 


the  sensory  olfactory  epithelium  (Fig.  352).  It  is  also  differentiated  by  the  de- 
velopment of  the  so-called  organ  of  Jacobson,  of  the  conchae,  of  the  ethmoidal 
cells  and  of  the  cranial  sinuses. 

The  Organ  of  Jacobson  is  a  rudimentary  epithelial  structure  which  first  ap- 
pears in  8.5  to  9  mm.  embryos  on  the  median  wall  of  the  nasal  fossa  (Fig.  350 
C,  E).  The  groove  deepens  and  closes  caudally  to  form  a  tubular  structure  in 
the  cranial  portion  of  the  nasal  septum  (Fig.  352).  At  two  and  a  half  months 
it  attains  a  length  of  0.42  mm.  It  is  supplied  by  nerve  fibers  which  arise  from 
cells  in  its  epithelium  and  in  part  by  the  n.  terminalis.  In  late  fetal  stages  it  often 
degenerates  but  may  persist  in  the  adult  (Merkel,  Mangakis).     Special  cartilages 


DEVELOPMENT  OF  THE  SENSE  ORGANS 


373 


are  developed  for  its  support  (Fig.  352).     The  organ  of  Jacobson  is  not  func- 
tional in  man  but  in  many  animals  constitutes  a  special  olfactory  organ. 

The  Conchae  are  structures  which  are  poorly  developed  in  man.  They  appear 
on  the  lateral  and  median  walls  of  the  primitive  nasal  fossae.  The  inferior  concha, 
or  maxillo-turbinal,  is  developed  first  in  human  embryos  (Figs.  352  and  353).  It 
forms  a  ridge  along  the  caudal  two-thirds  of  the  lateral  wall  and  is  marked  off  by  a 
ventral  groove  which  becomes  the  inferior  nasal  meatus.  The  naso-turb'uud  is 
very  rudimentary  and  appears  after  the  fourth  month  as  a  slight  elevation  dorsal 
and  cranial  to  the  inferior  concha  (Fig.  354).  Dorsal  to  the  inferior  concha 
arise  five  ethmo-turbinals,  which  grow  smaller  and  are  located  more  caudad  as  we 
pass  from  the  first  to  the  fifth  (Fig.  354).     According  to  Peter,  the  ethmo-tur- 


Horizontal  ethmoid  plate 


Hard  palate 


Upper  Up 


lateral  lame/la 
Sphenoid 

Choana 
Tube 


Soft  palate 


Fig.  354. — Right  nasal  passage  of  a  fetus  at  term  (Killian).     I,  maxillo-turbinal :  II-IY, 

ethmo-turbinals. 

binals  arise  on  the  median  wall  of  the  nasal  fossa  and,  by  a  process  of  unequal 
growth,  are  transferred  to  the  lateral  wall  (Fig.  353).  Accessory  conchae  are 
also  developed,  according  to  Killian. 

In  addition  to  the  ridges  formed  by  the  conchae,  there  are  developed  in  the  grooves  between 
the  ethmo-turbinals  the  ethmoidal  cells.  The  frontal  recess  gives  rise  to  the  frontal  sinus.  At 
the  middle  of  the  third  month  the  maxillary  sinus  grows  out  from  the  inferior  recess  of  the  first 
groove.  The  most  caudal  end  of  the  nasal  fossa  becomes  the  sphenoidal  sinus,  which,  as  it 
increases  in  size,  invades  the  sphenoid  bone. 

The  cells  of  the  olfactory  epithelium  become  ciliated  but  only  a  small  area,  representing 
the  primitive  epithelial  invagination,  functions  as  an  olfactory  sense  organ.  The  olfactory  cells 
of  this  area  give  rise  to  the  olfactory  fibers  which  constitute  the  nerve.  The  development  of 
this  has  been  described  on  page  355. 

IV.    The  Development  of  the  Eye 
The  anlage  of  the  human  eye  appears  in  embryos  of  2.5  mm.  as  a  thickening 
and  evagination  of  the  neural  plate  of  the  fore-brain.     At  this  stage  the  neural 


374 


THE   PERIPHERAL   NERVOUS   SYSTEM 


groove  of  the  fore-brain  has  not  closed  (Fig.  312).  At  4  mm.  the  optic  vesicles 
are  larger  but  still  may  be  connected  by  a  wide  opening  with  the  brain  cavity 
(Fig.  355  A,  B).  In  the  section  shown  in  Fig.  355  C,  the  optic  vesicle  is  at- 
tached to  the  brain  wall  by  a  distinct  optic  stalk. 

The  thickening,  flattening  and  invagination  of  the  distal  and  ventral  wall  of 
the  optic  vesicle  gives  rise  to  the  optic  cup  (Fig.  355  B,  C,  D).  The  area  of  in- 
vagination extends  ventrally  along  the  optic  stalk  and  produces  the  chorioid 
fissure  of  the  optic  cup  (Figs.  356  and  358). 

At  the  same  time  that  the  optic  vesicle  is  converted  into  the  optic  cup,  the 


Optic  vesicle 
Optic  sfalK 

C 


Forebrain 
Optic  vesicle 


Anlaqe 
fLens 


Optic  vesicle 

Bet  in  a  I  layer 
D 


Fig.  355- — Stages  in  the  early  development  of  the  eye.    A,  B,  at  4  mm.;  C,  at  5  mm.;  D,  at  6.25  mm. 

(after  Keibel  and  Elze). 


ectoderm  overlying  the  former  thickens,  as  seen  in  Fig.  355  B,  forming  the  lens 
plate,  or  optic  placode.  This  plate  invaginates  to  form  the  lens  pit,  the  external 
opening  of  which  closes  in  embryos  of  6  to  7  mm.  (Fig.  355  D),  producing  the 
lens  vesicle  which  remains  at  first  attached  to  the  overlying  ectoderm.  In  an 
embryo  of  10  mm.  (Fig.  357)  the  lens  vesicle  has  separated  from  the  ectoderm, 
which  will  form  the  epithelium  of  the  cornea.  The  lens  vesicle  in  earlier  stages 
is  closely  applied  to  the  inner  wall  of  the  optic  cup,  but  now  it  has  separated  from 
it,  leaving  a  space  in  which  the  vitreous  body  is  developing.  The  inner  retinal 
layer  of  the  optic  cup  has  become  very  thick  and  is  applied  to  its  outer  layer,  so 


DEVELOPMENT    OF    TIIK    SKNSK    ORGANS 


375 


Diencepha/on  _£ 


that  the  cavity  of  the  primitive  optic  vesicle  is  nearly  obliterated.  Pigment 
granules  have  begun  to  appear  in  the  outer  cells  which  form  the  pigment  layer 
of  the  retina.  Mesenchymal 
tissue  surrounds  the  optic  cup 
and  is  beginning  to  make  its 
way  between  the  lens  vesicle 
and  the  ectoderm.  Here  is  later 
developed  the  anterior  chamber 
of  the  eye  as  a  cleft  in  the  meso- 
derm. The  distal  mesenchy- 
mal tissue  (next  the  ectoderm) 
forms  the  substantia  propria  of 
the  cornea  and  its  posterior 
epithelium,  while  the  proximal 
mesenchyma    (next     the     lens) 

differentiates  into  the  vascular  capsule  of  the  lens.     The  mesenchyme  surround- 
ing the  optic  cup  is  continuous  with  that  which  forms  the  cornea  and  later 

Lens  I  vesicle  ,1/iTreous  body         j  Upti 

Mese, 


Crystalline  la 


Choriod  fissure 


Optic ^  stalk 
Fig.  356. — The  optic  stalk,  cup  and  lens  of  an  embryo 
of  twenty-seven  days.     On  the  ventral  surface  of  the  optic 
cup  is  seen  the  chorioid  fissure  of  the  primitive  eye  (from 
Fuchs,  after  Hochstetter  in  Kollmann's  Handatlas). 


EpiThelium 
of  Cornea 


> Pigment  lay' 
of  retina 


Nervous    layer 
of  retina 


Fig.  357. — A  transverse  section  through  the  optic  cup,  stalk  and  lens  of  a  10  mm.  human  embryo.    X  100. 


gives  rise  to  the  sclerotic  layer,  to  the  chorioid  layer  and  to  the  anterior  layers 
of  the  ciliary  body  and  iris. 


376 


THE    PERIPHERAL   NERVOUS    SYSTEM 


Both  the  inner  and  outer  layers  of  the  optic  cup  are  continued  into  the  optic 
stalk,  as  seen  in  Fig.  357.  This  is  due  to  the  invagination  of  the  ventral  wall  of 
the  optic  stalk  and  the  formation  in  it  of  the  chorioid  fissure  when  the  optic  vesicle 
is  transformed  into  the  optic  cup  (Fig.  356).  Into  the  chorioid  fissure  grows  the 
central  artery  of  the  retina,  and  carries  with  it  into  the  posterior  cavity  of  the 
eye  a  small  amount  of  mesenchyme,  as  seen  in  the  eye  of  a  12  mm.  embryo  (Fig. 
358).  Branches  from  this  vessel  extend  to  the  posterior  surface  of  the  lens  and 
supply  it  with  nutriment  for  its  growth.  At  a  later  stage  the  chorioid  fissure 
closes,  so  that  the  distal  rim  of  the  optic  cup  forms  a  complete  circle. 


1  Ectoderm 


I  Epithet 7  at  laiyeroflens 

^lament  Layer  of  the  retina. 

Nervous  layer  of  retina. 

Chorioid  fissure 


Central  artery 
Vitreous  body 
Layer   of  lens  fibers 


Mesenchyme 


Fig.  358. — Transverse  section  passing  through  the  optic  cup  at  the  level  of  the  chorioid  fissure. 
The  central  artery  of  the  retina  is  seen  entering  the  fissure  and  sending  a  branch  to  the  proximal  surface 
of  the  lens;  from  a  12.5  mm.  embryo.     X  105. 


If  the  chorioid  fissure  fails  to  close,  the  optic  cup  remains  open  at  one  point  and  this 
results  in  the  defective  development  of  the  iris,  ciliary  body  and  chorioid  layer.  Such  defects 
are  known  as  coloboma. 

It  was  formerly  supposed  that  the  development  of  the  lens  vesicle  caused  the  formation  of 
the  optic  cup  by  pushing  in  its  distal  wall.  It  has  been  shown  by  W.  H.  Lewis  that  this  is  not 
the  case,  for  if  an  anlage  of  the  optic  vesicle  from  an  amphibian  embryo  is  transplanted  to  some 
other  part  of  the  embryo,  it  will  not  only  develop  into  an  optic  cup,  but  the  ectoderm  over 
it  will  differentiate  a  lens  vesicle. 

The  lens  vesicle  in  its  early  development  from  the  ectoderm  has  been  de- 
scribed. Its  proximal  wall  is  much  thickened  in  10  mm.  embryos  and  these  cells 
form  the  lens  fibers  (Fig.  357).     A  few  cells  early  separated  off  from  the  wall  of 


DEVELOPMENT  OF  THE  SENSE  ORGANS 


377 


the  lens  pit  are  enclosed  in  the  vesicle  and  have  degenerated  in  12.5  mm.  embryos 
(Fig.  358).  At  this  stage  the  lens  fibers  of  the  proximal  wall  are  longer  and  this 
layer  will  soon  obliterate  the  cavity  of  the  vesicle,  as  in  embryos  of  15  to  17  mm. 
(Fig.  359).  The  cells  of  the  distal  layer  remain  of  a  low  columnar  type  and  con- 
stitute the  epithelial  layer  of  the  lens.  When  the  lens  fibers  attain  a  length  of 
0.18  mm.  they  cease  forming  new  fibers  by  cell  division.  New  fibers  thereafter 
arise  from  the  cells  of  the  epithelial  layer  at  its  line  of  union  with  the  lens  fibers. 
The  nuclei  are  arranged  in  a  layer  convex  toward  the  outer  surface  of  the  eye  and 
later  degenerate,  the  degencra- 


.EpitheliaL  layer 


Capsule 


Vascular 
•^    membrane 


Lens  fibers 


tion  beginning  centrally.  The 
structureless  capsule  of  the  lens 
is  probably  derived  from  the 
lens  cells.  Proximal  and  distal 
lens  sutures  are  formed  when 
the  longer  peripheral  fibers  over- 
lap the  ends  of  the  shorter  cen- 
tral fibers.  These  are  later  trans- 
formed into  "lens-stars"  (Fig. 
360).  The  lens,  at  first  some- 
what triangular  in  cross  section, 
becomes  nearly  spherical  at 
three  months  (Fig.  360). 

The  origin  of  the  vitreous 
body  has  been  in  doubt,  one 
view  deriving  it  from  the  mesen- 
chyma  which   enters   the  optic 

cup  through  the  chorioid  fissures  and  about  the  edge  of  lens,  another  view  hold- 
ing that  it  arises  from  cytoplasmic  processes  of  cells  in  the  retinal  layer. 

It  is  certain  that  the  vitreous  tissue  is  formed  before  mesenchyma  is  present  in  the  cavity 
of  the  optic  cup.  Szily  regards  this  primitive  vitreous  body  as  a  derivative  of  both  retinal  and 
lens  cells,  it  forming  a  non-cellular  network  of  cytoplasmic  processes  which  are  continuous 
with  the  cells  of  the  lens  and  retina.  With  the  ingrowth  of  the  central  artery  of  the  retina, 
from  which  the  artery  of  the  lens  passes  to  and  branches  on  the  proximal  surface  of  the  lens, 
a  certain  amount  of  mesenchymal  tissue  invades  the  optic  cup  and  this  tissue  probably  con- 
tributes to  the  development  of  the  vitreous  body  (Fig.  358). 

The  vitreous  body  may  therefore  be  regarded  as  a  derivative  both  of  the  ectoderm  and 
of  the  mesoderm. 


'  Ectoderm 

Fig.  359. — Section  through  the  lens  and  corneal  ecto- 
derm of  a  16  mm.  pig  embryo.     X  140. 


The  mesenchyma  accompanying  the  vessels  to  the  proximal  surface  of  the 


378 


THE    PERIPHERAL   NERVOUS    SYSTEM 


lens,  and  that  on  its  distal  surface,  give  rise  to  the  vascular  capsule  of  the  lens 
(Fig.  358).  On  the  distal  surface  of  the  lens  this  is  supplied  by  branches  of  the 
anterior  ciliary  arteries  and  is  known  as  the  pupillary  membrane.  The  vessels  in 
this  disappear  and  it  degenerates  just  before  birth.  The  artery  of  the  lens  also 
degenerates,  its  wall  persisting  as  the  transparent  hyaloid  canal.  Fibrillar  ex- 
tending in  the  vitreous  humor  from  the  pars  ciliata  of  the  retinal  layer  to  the  cap- 
sule of  the  lens  persist  as  the  zonula  ciliata  or  suspensory  ligament  of  the  lens. 


Anterior  epithelium 
of  cornea       \ 


■■r/f*1:  ■'./.■'■<■: 


Raphe  between 
palpebral 


Posterior  epithelium 
of  cornea 


\\ 


Epithelium 
of  lens 


Pars  iridica 
retina 

Pigment  layer 
of  retina 

Pars  optica 

retina 

Lens  fibers 
Lens  capsule 


Vitreous 
body 


Xarw.ne.H;l\. 


Fig.  360. — Section  through  the  distal  half  of  the  eyeball  and  through  the  eyelids  of  a  65  mm.  embryo. 

X35- 


Differentiation  of  the  Optic  Cup. — We  have  seen  that  of  the  two  layers  of  the 
optic  cup  the  outer  becomes  the  pigment  layer  of  the  retina.  Pigment  granules 
appear  in  its  cells  in  embryos  of  7  to  9  mm.  and  the  pigmentation  of  this  layer  is 
marked  in  12  mm.  embryos  (Fig.  358). 

The  inner  layer  of  the  optic  cup  is  the  retinal  layer  and  is  subdivided  into  a 
distal  zone,  the  pars  cceca,  which  is  non-nervous,  and  into  the  pars  optica,  or  the 


DEVELOPMENT  OF  THE  SENSE  ORGANS 


379 


nervous  retina  proper.  The  line  of  demarcation  between  the  pars  optica  and  the 
pars  ca?ca  is  a  serrated  circle,  the  ora  scrrala.  The  Mind  portion  of  the  retinal 
layer,  the  pars  caeca,  with  the  development  of  the  ciliary  bodies  is  differentiated 
into  a  pars  ciliaris  and  pars  iridis  retina.  The  former,  with  a  corresponding  zone 
of  the  pigment  layer,  covers  the  ciliary  bodies.  The  pars  iridis  forms  the 
proximal  layer  of  the  iris  and  blends  intimately  with  the  pigment  layer  in  this 
region,  its  cells  also  becoming  heavily  pigmented  (Fig.  360). 

The  pars  0 plica,  or  nervous  portion  of  the  retina,  begins  to  differentiate  prox- 
imally,  the  differentiation  extending  distally.  An  outer  cellular  layer  and  an 
inner  fibrous  layer  may  be  distinguished  in  12  mm.  embryos  (Fig.  358).  These 
correspond  to  the  cellular  layer  (ependymal  and  mantle  zones)  and  marginal 


Cone  cell 
Hod  cell 

Hod  cell 

Fiber o f 
Mueller. 
Arnaerin 
cell 

Ganglion 
cell 

Optic    - 
fibers 


Fig.  361. — Section  of  the  nervous  layer  of  the  retina  from  a  65  mm.  embryo 

figure  shows  diagrammatically  the  cellular  elements  of  the  retina  according  to  Cajal 


External  limit- 
ing membrane 

Layer  of  rod 
and  cone  cells 


Ganglionic 
layer 


rous  layer 


Internal  limit- 
ing membrane 

The  left  portion  of  the 
X  440. 


layer  of  the  neural  tube.  In  embryos  of  65  mm.  the  retina  shows  three  layers, 
large  ganglion  cells  having  migrated  in  from  the  outer  cellular  layer  of  rods 
and  cones  (Fig.  361).  In  a  fetus  of  the  seventh  month  all  the  layers  of  the  adult 
retina  may  be  recognized  (Fig.  362).  As  in  the  wall  of  the  neural  tube,  there  are 
differentiated  in  the  retina  supporting  tissue  and  nervous  tissue.  The  supporting 
elements,  or  fibers  of  Mueller,  resemble  ependymal  cells  and  are  radially  arranged 
(Figs.  361  and  362).  Their  terminations  form  internal  and  external  limiting 
membranes. 

The  neuroblasts  of  the  retina  differentiate  into  an  outer  layer  of  rod  and 
cone  cells,  the  visual  cells  of  the  retina  (Fig.  362).  Internal  to  this  layer  are  layers 
of  bipolar  and  multipolar  cells.  The  inner  layer  of  multipolar  cells  constitutes 
the  ganglion  cell  layer.     Axons  from  these  cells  form  the  inner  nerve  fiber  layer 


38o 


THE   PERIPHERAL   NERVOUS    SYSTEM 


of  optic  fibers.  These  converge  to  the  optic  stalk  and  grow  back  in  its  wall  to 
the  brain.  The  cells  of  the  optic  stalk  are  converted  into  neuroglia  supporting 
tissue  and  the  cavity  of  the  stalk  is  gradually  obliterated.  The  optic  stalk  is 
thus  transformed  into  the  optic  nerve  (see  p.  356). 

The  Sclerotic  and  Chorioid  Layers,  and  their  Derivatives. — After  the  mes- 
enchyme grows  in  between  the  ectoderm  and  the  lens  (Fig.  358)  the  lens  and  op- 
tic cup  are  surrounded  by  a  condensed  layer  of  mesenchymal  tissue,  which  gives 
rise  to  the  supporting  and  vascular  layers  of  the  eyeball.     By  condensation  and 

differentiation  of  its  outer  layers,  a 


mmm&sSM 


mm 


dense  layer  of  white  fibrous  tissue  is 
developed,  which  forms  the  sclerotic 
layer.  This  corresponds  to  the  dura 
mater  of  the  brain.  In  the  mesen- 
chyme of  25  mm.  embryos  a  cavity 
appears  distally,  which  separates 
the  condensed  layer  of  mesenchyme 
continuous  with  the  sclerotic  from 
the  vascular  capsule  of  the  lens  (Fig. 
360).  This  cavity  is  the  anterior 
chamber  of  the  eye  and  separates 
the  anlage  of  the  cornea  from  the 
lens  capsule.  An  inner  layer  of 
mesenchyme,  between  the  anlage 
of  the  sclerotic  and  the  pigment 
layer  of  the  retina,  becomes  highly 
vascular  during  the  sixth  month. 
Its  cells  become  stellate  in  form  and 
pigmented  so  that  the  tissue  is  loose 
and  reticulate.  This  vascular  tissue  constitutes  the  chorioid  layer  in  which  course 
the  chief  vessels  of  the  eye.  The  chorioid  layer  corresponds  to  the  pia  mater  of 
the  brain.  Distal  to  the  ora  serrata  of  the  retinal  layer  the  chorioid  is  differen- 
tiated: (1)  Into  the  vascular  folds  of  the  ciliary  bodies;  (2)  into  the  smooth 
fibers  of  the  ciliary  muscle;  (3)  into  the  stroma  of  the  iris.  The  proximal  pig- 
mented layers  of  the  iris  are  derived  from  the  pars  iridis  retina;  and  from  a  cor- 
responding zone  of  the  pigment  layer.  Of  these  the  pigment  layer  cells  give 
rise  to  the  sphincter  and  dilator  muscles  of  the  iris.  These  smooth  muscle  fibers 
are  thus  of  ectodermal  origin. 


Pigment  layer 
Bods  and  Cones 

Outer  nuclear  layer 

Outer  reticular  I  aver 
Inner  nuclear  layer 

ntier  reticular  layer 
Ganglion  cell  layer 


I — Nerve  fiber  layer 
Tibers  of  Mueller 


Infernal  limiting 
membrane 

Fig.  362. — Section  through  the  pars  optica  of  the 
retina  from  a  seven  months'  fetus.     X  440. 


DEVELOPMENT  OF  THE  SENSE  ORGANS  381 

The  Eyelids  appear  as  folds  of  the  integument  in  20  mm.  embryos.  The  lids 
come  together  and  the  epidermis  at  their  edges  is  fused  in  33  mm.  embryos  (Fig. 
360).  Later,  when  the  epidermal  cells  are  cornified  separation  of  the  eyelids 
takes  place.  The  epidermis  of  the  eyelids  forms  a  continuous  layer  on  their 
inner  surfaces  as  the  conjunctiva,  which  in  turn  is  continuous  with  the  anterior 
epithelium  of  the  cornea. 

The  Eyelashes,  or  cilia,  develop  like  ordinary  hairs  and  are  provided  with 
small  sebaceous  glands.  In  the  tarsus,  or  dense  connective  tissue  layer  of  the 
eyelids,  which  lies  close  to  the  conjunctival  epithelium,  there  are  developed  about 
30  tarsal  (Meibomian)  glands.  These  arise  as  ingrowths  of  the  epithelium  at  the 
edges  of  the  eyelids,  while  the  latter  are  still  fused. 

The  Lachrymal  Glands  appear  in  embryos  of  22  to  26  mm.,  according  to 
Keibel  and  Elze.  They  arise  as  five  or  six  ingrowths  of  the  conjunctiva,  dorsally 
and  near  the  external  angle  of  the  eye.  The  anlages  are  at  first  knob-like  and 
rapidly  lengthen  into  solid  epithelial  cords.  They  begin  to  branch  in  30  mm. 
embryos.  At  stages  between  40  and  60  mm.  additional  anlages  appear  which 
also  branch. 

In  3S  mm.  embryos  a  septum  begins  to  divide  the  gland  into  orbital  and  palpebral  por- 
tions. This  septum  is  complete  at  60  mm.,  the  five  or  six  anlages  first  developed  constituting 
the  orbital  part.  Lumina  appear  in  the  glandular  cords  in  embryos  of  50  mm.  by  the  degener- 
ation of  the  central  cells.  Accessory  lachrymal  glands  appear  in  30  cm.  fetuses.  The  lachrymal 
gland  is  not  fully  developed  at  birth,  being  only  one-third  the  size  of  the  adult  gland.  In  old 
age  marked  degeneration  occurs. 

The  Naso-lachrymal  Duct  is  formed  as  a  solid  epithelial  outgrowth  from  the 
conjunctiva  of  the  lachrymo-nasal  groove  at  the  internal  angle  of  the  eye.  The 
anlage  grows  down  through  the  mesenchyme  to  the  nasal  cavity.  The  lachrymal 
canals  are  budded  out  from  the  solid  anlage  of  the  lachrymal  duct  and  become 
connected  secondarily  with  the  inner  margins  of  the  palpebral.  The  primitive 
connection  of  the  lachrymal  duct  with  the  conjunctiva  is  lost.  The  anlage  of 
the  duct  appears  in  10  mm.  embryos  and  in  25  mm.  embryos  has  not  yet  reached 
the  nasal  cavity.     A  lumen  appears  in  the  duct  during  the  third  month. 

The  Development  of  the  Ear 
The  human  ear  consists  of  a  sound-conducting  apparatus  and  of  a  receptive 
organ.  The  conveyance  of  sound  is  the  function  of  the  external  and  middle  cars. 
The  end  organ  proper  is  the  inner  car  with  the  auditory  apparatus  residing  in  the 
cochlear  duct.  Besides  this  acoustic  function  the  labyrinthine  portion  of  the  inner 
ear  acts  as  an  organ  of  equilibrium. 


;82 


THE    PERIPHERAL   NERVOUS    SYSTEM 


The  Inner  Ear. — The  epithelium  of  the  internal  ear  is  derived  from  the  ecto- 
derm. Its  first  anlage  appears  in  embryos  of  2  mm.  as  thickened  ectodermal 
plates,  the  auditory  placodes  (Fig.  363  A).  These  are  developed  dorsal  to  the 
second  branchial  grooves  at  the  sides  of  the  hind-brain  opposite  the  fifth  neuro- 
meres  (Fig.  364) .    The  placodes  are  invaginated  to  form  hollow  vesicles  which  close 


Otic  vesicle 


Auditory  ganglion 

Auditory  placode 


Hind-brain 


Otic    vesicle 


Fig.  363. — Two  stages  in  the  early  development  of  the  internal  ear.  A,  section  through  the  head 
of  a  2  mm.  embryo  showing  the  auditory  placode  and  otic  vesicles;  B,  section  through  the  hind-brain 
and  otic  vesicles  of  an  early  human  embryo  (Keibel  and  Elze). 


Ectoderm 


Wall  of  hind  bra-m 


Near,  t 


Neur.5 


Fig.  364. — Four  sections  through  the  right  otic  vesicle  of  an  early  human  embryo,  r.  e.,  endo- 
lymphatic recess,  the  anlage  of  the  endolymph  duct  and  sac;  0.  v.,  otic  vesicle;  Neur.  4,  Near.  5,  neuro- 
meres  four  and  five  of  the  myelencephalon  (Keibel  and  Elze).     About  30  diameter. 


in  embryos  of  2.5  to  3  mm.,  but  remain  attached  to  the  ectoderm  for  some  time 
(Fig.  363  B). 

The  auditory  vesicle  or  otocyst  when  closed  and  detached  is  nearly  spherical, 
but  at  the  point  where  it  was  attached  to  the  ectoderm  a  recess  is  formed.  The 
point  of  origin  of  this  recess  is  shifted  later  from  a  dorsal  to  a  mesial  position  and 
it  constitutes  the  ductus  endolymphaticus  (Figs.  365  and  366  a).    The  endolymph 


DEVELOPMENT  OF  THE  SENSE  ORGANS 


383 


Wa.ll  of 

muelnncepha/on 


Cndolymph  Juct 


\/estibult 


far  an  Icy  e 


duct  corresponds  to  that  of  selachian  fishes,  which  remains  open  to  the  exterior. 
In  man,  its  dorsal  extremity  is  closed  and  dilated  to  form  the  endolymphatic  sac 
(Fig.  366  e). 

The  differentiation  of  the  auditory  vesicle  has  been  described  by  His,  Jr. 
and  more  recently  by  Streeter  (Amer.  Jour.  Anat.,  vol.  6,  1906).  In  an  em- 
bryo of  about  7  mm.  the  vesicle  has  elongated,  its  narrower  ventral  process  con- 
stituting the  anlage  of  the  cochlear  duel  (Fig.  366  a).  The  wider  dorsal  portion 
of  the  otocyst  is  the  vestibular  anlage  and  it  shows  indications  dorsally  of  the  de- 
veloping semicircular  canals.  These  are  formed  in  11  mm.  embryos  as  two 
pouches,  the  anterior  and  posterior 
canals  from  a  single  pouch  at  the 
dorsal  border  of  the  otocyst,  the 
external  canal  later  from  a  lateral 
outpocketing  (Fig.  366  d).  The  mar- 
gins of  these  pouches  are  thickened, 
but  elsewhere  their  walls  are  flattened 
together  and  fused  to  form  an  epi- 
thelial plate.  Three  such  epithelial 
plates  are  produced  and  internally 
about  the  periphery  of  each  plate 
canals  are  left  communicating  with 
the  cavity  of  the  vestibule.  Soon  the 
epithelial  plates  are  resorbed,  leaving 
spaces  between  the  semicircular  epi- 
thelial canals  and  the  vestibule  (Fig. 
366  c).     Dorsally  a  notch  separates 

the  anterior  and  posterior  canals.  Of  these  the  anterior  is  completed  first,  next 
the  posterior  canal.     The  external  canal  is  the  last  to  develop. 

In  a  20  mm.  embryo  (Fig.  366  e)  the  three  canals  are  present  and  the  coch- 
lear duct  has  begun  to  coil  like  a  snail  shell.  It  will  be  seen  that  the  anterior  and 
posterior  canals  have  a  common  opening  dorsally  into  the  vestibule,  while  their 
opposite  ends  and  the  cranial  end  of  the  external  canal  are  dilated  to  form 
ampulla.  In  each  ampulla  is  located  an  end  organ,  the  crista  acustica,  which  will 
be  referred  to  later.  By  a  constriction  of  its  wall  the  vestibule  is  differentiated 
into  a  dorsal  portion,  the  utriculus,  to  which  are  attached  the  semicircular  canals, 
and  a  ventral  portion,  the  sacculus,  which  is  connected  with  the  cochlear  duct 
(Fig.  366  e,f).     At  30  mm.  the  adult  condition  is  more  nearly  attained.     The 


Cochlear  anlage 


■'•V  V; *'' .  •  •*  •  *•  •  ■<*l«\*v« 


Fig.  365. — Transverse  section  through  the 
right  half  of  the  hind-brain  and  through  the  right 
otic  vesicle  showing  the  position  of  the  endolym- 
phatic duct.     From  an  embryo  6.9  mm.  long  (His). 


384  THE   PERIPHERAL   NERVOUS    SYSTEM 

sacculus  and  utriculus  are  more  completely  separated,  the  canals  are  relatively 
longer,  their  ampullae  more  prominent  and  the  cochlear  duct  is  coiled  about  two 
and  a  half  turns  (Fig.  366/).  In  the  adult,  the  sacculus  and  utriculus  become 
completely  separated  from  each  other,  but  each  remains  attached  to  the  endo- 
lvmph  duct  by  a  slender  canal  which  represents  the  prolongation  of  their  re- 
spective walls.  Similarly,  the  cochlear  duct  is  constricted  from  the  sacculus, 
the  basal  end  of  the  former  becomes  a  blind  process  and  a  canal,  the  ductus 
reuniens,  connects  the  cochlear  duct  with  the  sacculus. 

The  epithelium  of  the  labyrinth  at  first  is  composed  of  a  single  layer  of  low 
columnar  cells.  At  an  early  stage,  fibers  from  the  acoustic  nerve  grow  between 
the  epithelial  cells  in  certain  regions  and  it  becomes  modified  to  produce  special 
sense  organs.  These  end  organs  are  the  crista,  acusticce  in  the  ampullae  of  the 
semicircular  canals;  the  macula  acusticce  in  the  utriculus  and  sacculus,  and  the 
spiral  organ  (of  Corti)  in  the  cochlear  duct. 

The  cristas  and  maculae  are  static  organs,  or  sense  organs  for  equilibrium.  In 
each  ampulla  transverse  to  the  long  axis  of  the  canal  the  epithelium  and  under- 
lying tissue  form  a  curved  ridge,  the  crista.  The  cells  of  the  epithelium  are 
differentiated:  (1)  Into  sense  cells  with  bristle-like  hairs  at  their  ends,  and  (2) 
into  supporting  cells.  About  the  bases  of  the  sensory  cells  branch  nerve  fibers 
from  the  vestibular  division  of  the  acoustic  nerve.  The  maculae  resemble  the 
cristae  in  their  development  save  that  larger  areas  of  the  epithelium  are  differ- 
entiated into  cushion-like  end  organs.  Over  the  maculae  concretions  of  lime  salts 
may  form  otoconia  which  remain  attached  to  the  sensory  bristles.  The  true  or- 
gan of  hearing,  the  spiral  organ,  is  developed  in  the  basal  epithelium  of  the  coch- 
lear duct,  basal  having  reference  here  to  the  base  of  the  cochlea.  The  develop- 
ment of  the  spiral  organ  has  been  studied  carefully  only  in  the  lower  mammals, 
in  the  pig  by  Shambaugh,  Hardesty  and  Prentiss.  In  pig  embryos  of  5  cm.  the 
basal  epithelium  is  thickened,  the  cells  becoming  highly  columnar  and  the  nuclei 
forming  several  layers.  In  later  stages,  7  to  9  cm.,  inner  and  outer  epithelial 
thickenings  are  differentiated,  the  boundary  line  between  them  being  the  future 
spiral  tunnel  (Fig.  367  A).  At  the  free  ends  of  the  cells  of  the  epithelial  swellings 
there  is  differentiated  a  cuticular  structure,  the  membrana  tectoria,  which  appears 
first  in  embryos  of  4  to  5  cm.  The  cells  of  the  inner  (axial)  thickening  give  rise  to 
the  epithelium  of  the  spiral  limbus,  to  the  cells  lining  the  internal  spiral  sulcus  and 
to  the  supporting  cells  and  inner  hair  cells  of  the  spiral  organ  (Fig.  367  B,  C).  The 
outer  epithelial  thickening  forms  the  pillars  of  Corti,  the  outer  hair  cells  and  sup- 
porting cells  of  the  spiral  organ.     Differentiation  begins  in  the  basal  turn  of  the 


endolymph 

vest  i  b. 
pouch  ' 


fat  qroove 
vesUb.p.       CSCSUP 


^^^■..coch. 
^^^  pouch 


c.sc.post.  absorpt  foci 

lat  groove 
C.Sc.lat. 


a.  6.6  mm  lateral 


cochlea 
D  9  mm.  lateral. 


C  .  unim  lateral 


e .  20mm  .ateral. 


f. 30Tnm.  lateral. 


Fig.  366. — Six  stages  in  the  development  of  the  interna]  ear.  The  figures  show  lateral  views  of 
models  of  the  membranous  labyrinth— a  at  6.6  mm.;  6,  at  9  mm.;  c  at  13  mm.;  d  at  11  mm.;  c  at  20 
mm.,  and/at  30  mm.  (Streeter)  (X  25).  The  colors  yellow  and  red  arc  used  to  indicate  respectively  the 
cochlear  and  vestibular  divisions  of  the  acoustic  nerve  and  its  ganglia,  absorp.  focus,  area  of  wall  where 
absorption  is  complete;  cms,  crus  commune;  c.sc.lal.,  ductus  semicircularis  lateralis;  c.  sc.  post.,  ductus 
semicircularis  posterior;  c.sc.sitp.,  ductus  semicircularis  superior;  cochlea,  ductus  cochlearis;  coch. 
pouch,  cochlear  anlage;  endolymph.,  appendix  endolymphaticus;  sacc,  sacculus;  sac.  cndoL,  saccus  en- 
dnlymphaticus;  sinus  tttrc.  la!.,  sinus  utriculi  lateralis;    utric,  utriculus. 


DEVELOPMENT  OF  THE  SENSE  ORGANS 


385 


n.coc 


n.cocW 


Fig.  367. — Three  stages  in  the  differentiation  of  the  basal  epithelium  of  the  cochlear  duct  to  form 
the  spiral  organ  (of  Corti),  internal  spiral  sulcus  and  labium  vestibularc.  A ,  section  through  the  cochlear 
duct  of  an  8.5  cm.  pig  fetus;  B,  the  same  from  a  20  cm.  fetus;  C,  from  a  30  cm.  fetus  (near  term). 
cp.  s.  s p.,  epithelium  of  spiral  sulcus;  h.c.,  hair  cells;  i.ep.c,  inner  epithelial  thickening;  i.h.c,  inner  hair 
cells;  i.pil.,  inner  pillar  of  Corti;  lab.  vest.,  labium  vestibulare;  limb,  sp.,  limbus  spiralis;  m.  bas.,  basilar 
membrane;  m.  ted.,  membrana  tectoria;  m.  vest.,  vestibular  membrane;  n.  cock.,  cochlear  division  of 
acoustic  nerve;  o.ep.c.,  outer  epithelial  thickening;  o.h.c,  outer  hair  cells;  s.sp.,  sulcus  spiralis;  sc.tymp., 
scala  tympani;  st.H.,  stripe  of  Hensen;  t.sp.,  spiral  tunnel. 
25 


386  THE   PERIPHERAL   NERVOUS    SYSTEM 

cochlea  and  proceeds  toward  the  apex.  The  internal  spiral  sulcus  is  formed  by 
the  degeneration  and  metamorphosis  of  the  cells  of  the  inner  epithelial  thicken- 
ing which  lie  between  the  labium  vestibulare  and  the  spiral  organ  (Fig.  367  B,  C). 
These  cells  become  cuboidal,  or  flat,  and  line  the  spiral  sulcus,  while  the  membrana 
tectoria  loses  its  attachment  to  them.  The  membrana  tectoria  becomes  thickest 
over  the  spiral  organ  and  in  full  term  fetuses  is  still  attached  to  its  outer  cells 
(Fig.  367  C). 

According  to  Hardesty  (Amer.  Jour.  Anat.,  vol.  8)  the  membrana  tectoria  is  not  devel- 
oped from  the  cells  of  the  spiral  organ  and  therefore  is  not  attached  to  it  at  any  time.  From  what 
is  known  of  the  development  of  the  spiral  organ  in  human  embryos,  it  follows  the  same  lines 
of  development  as  described  for  the  pig.  It  must  develop  relatively  late,  however,  for  in  the 
cochlear  duct  of  a  new-born  child  figured  by  Krause  the  spiral  sulcus  and  the  spiral  tunnel  are  not 
yet  present. 

The  mesenchyme  surrounding  the  labyrinth  is  differentiated  into  a  fibrous 
membrane  directly  surrounding  the  epithelium,  and  into  the  perichondrium  of 
the  cartilage  which  develops  about  the  whole  internal  ear.  Between  these  two 
is  a  more  open  mucous  tissue  which  largely  disappears,  leaving  the  perilymph 
space.  The  membranous  labyrinth  is  thus  suspended  in  the  fluid  of  the  peri- 
lymph space.  The  bony  labyrinth  is  produced  by  the  conversion  of  the  cartilage 
capsule  into  bone.  In  the  case  of  the  cochlea,  large  perilymph  spaces  form  above 
and  below  the  cochlear  duct.  The  duct  becomes  triangular  in  section  as  its 
lateral  wall  remains  attached  to  the  bony  labyrinth,  while  its  inner  angle  is  ad- 
herent to  the  modiolus.  The  upper  perilymph  space  is  formed  first  and  is  the 
scala  vestibuli,  the  lower  space  is  the  scala  tympani.  The  thin  wall  separating 
the  cavity  of  the  cochlear  duct  from  that  of  the  scala  vestibuli  is  the  vestibular 
membrane  (of  Reissner).  Beneath  the  basal  epithelium  of  the  cochlear  duct  a 
fibrous  structure,  the  basilar  membrane,  is  differentiated  by  the  mesenchyme. 
The  modiolus  is  not  preformed  as  cartilage,  but  is  developed  directly  from  the 
mesenchyme  as  a  membrane  bone.  The  development  of  the  acoustic  nerve  has 
been  described  on  page  356  with  the  other  cerebral  nerves. 

The  Middle  Ear. — The  middle  ear  cavity  is  differentiated  from  the  first 
pharyngeal  pouch  which  appears  in  embryos  of  3  mm.  The  pouch  enlarges  rap- 
idly up  to  the  seventh  week,  is  flattened  horizontally  and  is  in  contact  with  the 
ectoderm.  During  the  latter  part  of  the  second  month,  in  embryos  of  24  mm., 
the  wall  of  the  tympanic  cavity  is  constricted  to  form  the  tubo-tympanic  (Eus- 
tachian) canal.  This  tube  lengthens  and  its  lumen  becomes  slit-like  during 
the  fourth  month.     The  tympanic  cavity  is  surrounded  by  loose  areolar  connec- 


DEVELOPMENT   OP    Till.    SENSE   ORGANS  387 

tive  tissue  in  which  the  auditory  ossi<  Irs  arc  developed  and  for  a  time  embedded. 
The  pneumatic  cells  arc  formed  at  the  close  of  fetal  life. 

The  development  of  the  auditory  ossicles  has  been  described  by  Broman 
(Verh.  Anat.  Gesellsch.,  Kiel,  Anat.  An/.  Suppl.,  vol.   14,  1898).     According 

to  his  account,  the  condensed  mesenchyma  of  the  first  and  second  branchial 
arches  gives  rise  to  the  ear  ossicles.  This  tissue  is  divided  in  the  proximal  part 
of  the  arches  into  lateral  and  median  masses. 

The  malleus  is  formed  from  the  distal  portion  of  the  median  mesenchymal 
mass  of  the  first  arch,  along  with  Meckel's  cartilage  of  the  mandible.  The 
cartilaginous  anlage  of  the  malleus  is  continuous  with  Meckel's  cartilage.  Be- 
tween it  and  the  incus  is  an  intermediate  disk  of  tissue,  which  later  forms  an 
articulation.  When  the  malleus  begins  to  ossify  it  separates  from  Meckel's 
cartilage. 

The  incus  is  derived  from  the  proximal  portion  of  the  lateral  mesenchymal 
mass  of  the  first  branchial  arch.  The  anlage  of  the  incus  unites  with  that  of  the 
capsule  of  the  labyrinth  and  separates  from  it  only  when  its  cartilage  develops. 
It  is  early  connected  with  the  anlage  of  the  stapes,  and  the  connected  portion  be- 
comes the  cms  longum.     Between  this  and  the  stapes  an  articulation  develops. 

The  stapes  and  Reichert's  cartilage  are  derived  from  the  median  mesenchy- 
mal mass  of  the  second  branchial  arch.  The  mesenchymal  anlage  of  the  stapes  is 
perforated  by  the  stapedial  artery,  and  its  cartilaginous  anlage  is  ring-shaped. 
This  form  persists  until  the  middle  of  the  third  month  when  it  assumes  its  adult 
structure  and  the  stapedial  artery  disappears. 

Fuchs,  after  studying  the  development  of  the  ear  ossicles  in  rabbit  embryos,  concludes: 
(1)  that  the  stapes  is  derived  from  the  capsule  of  the  labyrinth;  (2)  that  the  malleus  and  incus 
arise  independent  of  the  first  branchial  arch. 

The  External  Ear. — The  external  ear  is  developed  from  and  about  the  first 
branchial  groove.  The  auricle  arises  from  sLx  elevations  which  appear  three  on 
the  mandibular  and  three  on  the  hyoid  arch  (Fig.  368).  These  anlages  were  first 
described  by  His. 

They  are  numbered  ventro-dorsally  on  the  mandible  and  in  the  reverse  direction  on  the 
hyoid  arch.  Caudal  to  the  hyoid  anlages  a  fold  of  the  integument  is  formed,  the  hyoid  helix 
or  auricular  fold.  A  similar  fold  forms  later  dorsal  to  the  first  branchial  groove  and  unites  with 
the  auricular  fold,  to  form  with  it  the  free  margin  of  the  auricle.  The  point  of  fusion  of  these 
two  folds  marks  the  position  of  the  satyr  tubercle,  according  to  Schwalbe.  Schwalbe  derives  the 
tragus  from  mandibular  hillock  1;  the  helix  from  mandibular  hillocks  2  and  3;  the  antihelix 
from  hyoid  hillocks  4  and  5;  the  antitragus  from  hyoid  hillock  6.    Darwin's  tubercle  appears  at 


388 


THE   PERIPHERAL   NERVOUS    SYSTEM 


about  the  middle  of  the  margin  of  the  free  auricular  fold,  and  corresponds  to  the  tip  of  the  mam- 
malian auricle. 

The  external  auditory  meatus  is  formed  as  an  ingrowth  of  the  first  branchial 
groove.  In  embryos  of  12  to  15  mm.  the  wall  of  this  groove  is  in  contact  dorsally 
with  the  entoderm  of  the  first  pharyngeal  pouch.  Later,  however,  this  contact 
is  lost,  and  during  the  latter  part  of  the  second  month,  according  to  Hammar, 


Fig.  368. — Six  stages  in  the  development  of  the  external  ear.  1,  2,  3,  elevations  on  the  mandibular 
arch;  4,  5,  6,  elevations  on  the  hyoid  arch.  1,  tragus;  2,  3,  helix;  4,  5,  antihelix;  6,  antitragus.  c, 
hyoid  helix  or  auricular  fold  (His  from  McMurrich's  "Human  Body").  A,  n  mm.;  B,  13.6  mm.;  C, 
15  mm.;  D,  beginning  of  third  month;  E,  fetus  of  85  mm.;  F,  fetus  at  term. 

an  ingrowth  takes  place  from  the  ventral  portion  of  the  groove,  to  form  a  funnel- 
shaped  canal. 

The  lumen  of  this  tube  is  temporarily  closed  during  the  fourth  and  fifth  months,  but  later 
re-opens.  During  the  third  month  a  plate  of  cells  at  the  extremity  of  the  primary  auditory 
meatus  grows  in  and  reaches  the  lower  wall  of  the  tympanic  cavity.  During  the  seventh  month 
a  space  is  formed  by  the  splitting  of  this  plate,  and  the  secondary  portion  of  the  meatus  is 
thus  developed. 

The  tympanic  membrane  is  formed  by  a  thinning  out  of  the  tissue  in  the  region 
where  the  wall  of  the  external  auditory  meatus  abuts  upon  the  wall  of  the  tym- 
panic cavity. 


NDEX 


Abdominal  pregnancy,  30 
Abortion,  So 
Acid  carmine,  48 

haematoxylin,  48 
Adipose  tissue,  294 
Alee  nasi,  154 
Allantoic  stalk,  76,  79,  152 

vessels,  76,  77 
Allantois,  67,  76,  77,  83,  92,  105,  119 

derivation  of,  79,  83 
Amboceptor,  30 
Ameloblasts,  163 
Amitosis,  22 

Amnion,  origin  of,  74,  83 
bat,  83 
chick,  65,  75 
human,  80,  83 

Pig,  77 
Amniota,  74 
Amphiaster,  22 
Amphioxus,  ^^ 

Ampulla  of  ductus  deferens,  226 
Anal  membrane,  169 
Angioblast,  48 
chick,  48 
human,  251 
Anlage  defined,  13,  48 
Annelids,  ^^ 
Annulus,  22 

Anomalies,  167,  177,  182,  186,  202,  213,  225 
Ansa  hypoglossi,  353,  358 
Anus,  105,  152,  169 
Aorta,  origin  of,  255 

chick,  52 

descending,  52,  92,  106,  137,  267 

dorsal,  56,  106,  107 

pig,  106,  107,  137 

ventral,  56,  267 
Aortic  arches,  92,  129,  137,  276 

chick,  67 

human,  92 

pig,  107,  129,  137 
Appendix  epididymis,  225 
testis,  226,  236 
vermiformis,  182 


Aqueduct,  cerebral,  336 
Archenteron,  38 
Archipallium,  344 
Arcuate  fibers,  $^^ 
Arcus  pharyngo-palatini,  158 
Area,  germinal,  36 

opaca,  43,  48 

pellucida,  43,  54 

vasculosa,  57 
Arteries,  allantoic,  76,  77 

basilar,  135,  272 

carotid,  107,  129,  270 

central,  of  retina,  376 

changes  at  birth,  286 

cceliac,  107,  129,  274 

epigastric,  274 

hepatic,  186 

hypogastric,  275 

iliac,  129,  275 

intercostal,  274 

internal  mammary,  274 

intersegmental,  107,  129,  268,  274 

mesenteric,  129 
inferior,  129,  269 
superior,  129,  269,  274 

of  extremities,  275 

of  pig,  106,  129 

ovarian,  274 

phrenic,  274 

pulmonary,  107,  129,  177,  265,  271 

renal,  213,  274 

spermatic,  274 

subclavian,  107,  129,  270,  274 

suprarenal,  274 

umbilical,  92,  107,  118,  144 

ventro-lateral,  107,  129 

vertebral,  129,  272 

vitelline,  57,  68,  92,  107,  118,  129,  268,  269 
Arytenoid  swellings,  101,  124,  160,  174 
Ascaris  megalocephala,  27 
Atrium,  67,  92,  258 
Auricle  of  ear,  387 

of  heart.     See  Atrium 
Autonomic  system,  364 
Axial  filament,  22 


389 


39° 


INDEX 


Basophile  (mast  leucocyte),  254 
Biogenesis,  law  of,  14 
Bladder,  85,  152,  213,  215 
Blastoccel,  33 
Blastoderm,  36 
Blastodermic  vesicle,  36 
Blastopore,  38 
Blastula,  33,  37 
Blood,  53 

cells,  251 

islands,  4S,  251 

plastids,  253 

plates,  254 
Blood-vessels,  changes  at  birth,  286 

chick,  53,  56,  67 

human,  92,  251,  255 

pig,  105,  128 

primitive,  267 
Body  cavities,  188 
Body-stalk,  80,  85 
Bone,  endochondral,  296 

growth  of,  297 

histogenesis  of,  295 

membrane,  295 

palatine,  158 
Brachial  plexus,  353 
Brachium  conjunctivum,  335 

pontis,  334 
Brain,  human,  325 

01  pig,  i23 
Branchial  arches,  chick,  67 
human,  89 
Pig,  97,  112 

clefts,  65,  97 

ducts,  170 

vesicles,  170 
Bulbar  swelling,  128 
Bulbo-urethral  glands,  235 
Bulbus  cordis,  67,  92 
Bursa  infracardiaca,  199 

omentalis,  197 


Cecum,  152,  182 

Calcar  avis,  349 

Canal,  atrio-ventricular,  106,  259 

Gartner's,  226,  237 

hyaloid,  378 

incisive  (of  Stenson),  157 

inguinal,  231,  232 

neurenteric,  41 

notochordal,  41 

pleuro-peritoneal,  191 

semicircular,  383 

tubo-tympanic,  387 


Canalis  epididymis,  225 
Capsule  of  Bowman,  205 

internal,  343 

of  liver,  200 
Cartilage,  arytenoid,  175 

corniculate,  175 

cricoid,  175 

cuneiform,  175 

histogenesis  of,  294 

Meckel's,  295 

of  epiglottis,  174 
Cauda  equina,  325 
Caudal  flexure,  89 
Caudate  lobe  of  liver,  201 

nucleus,  343 
Caul,  83 
Cavity,  body,  188 

oral,  151 

pericardial,  63,  188,  190 

peritoneal,  63,  194,  195 

pleural,  63,  194,  195 

pleuro-pericardial,  50,  190 

pleuro-peritoneal,  188,  190 

tympanic,  170,  386 
Cells,  germ,  17 

giant,  254 

lutein,  224 

sustentacular,  221 

taste,  369 
Centra  of  vertebrae,  151 
Central  nervous  system,  319 
chick,  49,  54,  66 
human,  89,  319 
pig,  122 
Centriole,  22 
Centrosome,  17 
Cephalic  flexure,  65,  87 
Cerebellum,  150,  334 
Cerebral  cortex,  349 

hemispheres,  147,  327,  346 

nerves.     See  Nerves,  123 
Chick  embryos,  43,  48 
preservation  of,  48 
study  of,  48,  54,  65 
Chin,  154 
Choanas,  371 
Chorda  dorsalis,  49.     See  Notochord 

gubernaculi,  230 
Chorioid  plexus,  150,  332,  336,  340 
Chorion,  origin  of,  74 
chick,  74,  75 
frondosum,  243 
human,  81 
keve,  243 
Pig,  77 


INDKX 


391 


Chorion,  villi  of,  cSo,  240 

Chromaffin  bodies,  300 

aortic,  366 
Chromatin,  17,  24 

Chromosomes,  23 
eu  cessory,  24,  32 
number  of,  24 

Circulation,  fetal,  284 
Circulatory  system,  92,  251 
Clava,  ^3 
Cleavage,  ss 
Clitoris,  152,  233,  237 
Cloaca,  oi,  104,  105,  119,  213 
Cloaca]  membrane,  105,  169,  213 

tubercle,  2^2 
Closing  plates,  103,  169 
Cochlea,  384 
Coelom,  45.  50,  190 

chick,  63 

extraembryonic,  188 

human,  188,  190 

umbilical,  182 
Collateral  eminence,  349 
Colliculus,  facial,  361 

inferior,  336 

superior,  336 
Coloboma,  376 
Colon,  142,  182,  183 
Column,  gray,  322 
Columna  nasi,  154 
Columns,  renal,  209 
Commissure,  anterior,  344,  345 

ganglionic,  364 

gray,  322 

hippocampal,  345 

posterior,  of  labia  majora,  233 

white,  322 
Conchae,  373 
Concrescence  theory,  41 
Coni  vasculosi.  2:5 
Conjunctiva,  381 
Copula,  102,  124,  160 
Cord,  spinal,  320 

nephrogenic,  205,  208 

umbilical,  79 
Cornea,  380 
Corona  radiata,  20 
Coronary  appendage,  194,  201 

sinus  of  heart,  279 

sulcus  of  phallus.  23a 
Corpora  quadrigemina,  328,  336 
Corpus  albicans,  224 

callosum,  344,  345 

haemorrhagicum,  224 

luteum,  224 


Corpus  striatum,  327,  339,  341 
Cortex  cerebri,  339,  349 

t  !re»  entic  groove,  40 
Crista  acustica,  383 
Crura  cerebri,  328 
Cryptorchism,  221,  231 

Cumulus  oophorus,  224 
Cuneus,  m,  349 
Cutis,  305 
plate,  119 


Decidua  basalis  (serotina),  239 
capsularis  (reflexa),  239,  244 

vera  (parietalis),  239,  243 
Decidual  cells,  244 

membranes,  239 
separation  of,  249 
Dentition,  167 
Dermatome,  299 
Dermis  (corium),  305 
Dermo-muscular  plate,  62 

Descending  spinal  tract  of  trigeminal  nerve,  359 
Diaphragm,  195 

dorsal  pillars  of,  194 

origin  of,  195 
Diaster,  23 

Diencephalon,  66,  89,  100,  336 
Differentiation  of  tissues,  13 
Digestive  canal,  chick,  55,  67 
human,  89 
pig,  6  mm.,  101 
10  mm.,  124 
glands,  89 
Dissecting  instruments,  145 
Dissections,  face,  153 

lateral,  99,  146 

median,  148 

palate,  155 

pig  embryos,  145 

tongue,  158 

ventral,  153 
Diverticulum,  cardiac,  127 

hepatic,  104 

Meckel's,  182 

of  pharyngeal  pouches,  103 
Ducts,  bile,  127 

branchial,  170 

cervical,  1 70 

cochlear,  383 

cystic,  184 

hepatic,  127,  184 

mesonephric,  92 

Muellerian,  218 

naso-lachrymal,  381 


392 


INDEX 


Ducts,  papillary,  208 

periportal,  185 

pronephric,  204 

thoracic,  287 
Ductuli  efferentes,  207,  225 
Ductus  arteriosus,  271,  286 

choledochus,  127,  1S5 

deferens,  226 

endolymphaticus,  382 

venosus,  108,  131,  278,  286 
Duodenum,  182 
Dyads,  27 


Ear,  external,  155,  3S7 

internal,  55,  3S2 

middle,  386 
Ectoderm,  37 

derivatives  of,  34 

formation  of,  37 
Elastic  tissue,  293 
Embryos,  chick,  43,  48,  54,  65 

human,  80.     See  Human  embryos 

pig,  6  mm.,  97 
10  mm.,  120 
dissection  of,  145 
Enamel  organ,  162 
Encephalon,  89 
Endocardial  cushion,  138,  259 
Endocardium,  139,  256 
Endothelium,  64 
End-piece,  22 
Enlargement,  cervical,  324 

lumbar,  324 
Entoderm,  37 

derivatives  of,  64 

histogenesis  of,  290 

origin  of,  38 
Entodermal  canal,  168 
Eosinophils,  254 
Ependymal  cells,  317 

layer,  133,  31' 
Epicardium,  52,  106, 139,  256 
Epidermis,  304 
Epididymis,  226 
Epigenesis,  12 

Epiglottis,  101,  124,  151,  160,  174 
Epiphysis  (pineal  body),  150,  336 
Epithalamus,  328 
Epithelial  bodies,  90,  172 
Epithelium,  64 
Epitrichium,  305 
Eponychium,  309 
Epoophoron,  226,  23 
Erythroblasts,  252 


Erythrocytes,  252,  253 

Esophagus,  89,  103,  127,  151,  177 

Ethmoidal  cells,  373 

Eustachian  tube,  90,  170 

External  auditory  meatus,  98,  120,  155 

Extraembryonic  mesoderm,  46,  80 

Extremity,  89,  98,  121 

Eye,  chick,  54,  66 

human,  89,  373 

anterior  chamber  of,  375 

pig,  97,  122 
Eyelashes,  381 
Eyelids,  381 


Face,  development  of,  153 
Facial  colliculus,  361 

nerve,  100,  123 
Falciform  ligament,  142 
Fasciculi  of  spinal  cord,  323 
Fasciculus,  median  longitudinal,  332 
Fertilization,  30 
Fetal  membranes,  human,  80 

pig,  77 
Fetus,  93 

Fibrin,  canalized,  248 
Filum  terminale,  324 
Fissure,  calcarine,  349 

chorioid,  of  cerebrum,  340 
of  eye,  374,  376 

collateral,  349 

great  longitudinal,  340 

hippocampal,  347 

lateral  (Sylvian),  347 

parietooccipital,  348 
Fixation  of  pig  embryos,  145 
Flagellum  of  spermatozoon,  21 
Flexures,  caudal,  89 

cephalic,  65,  89,  325 

cervical,  147,  326 

dorsal,  147 

pontine,  147,  326 
Flocculus,  334 
Floor  plate,  321,  330 
Foramen,  epiploic  (of  Winslow),  142,  198,  199 

interatrial,  106,  260 

interventricular,  of  heart,  265,  266 
Monroi,  328,  340 

ovale,  106,  138,  260,  263,  286 
Fore-brain,  chick,  49,  53 

human,  319 
Fore-gut,  chick,  51,  67 

human,  168 
Fornix,  344 
Fossa,  incisive,  157 


INDEX 


595 


Fossa  ovalis,  264 
Fovea  cardiaca,  48,  51 
Frenulum  «>f  prepuce,  234 
Funiculi  of  spinal  cord,  32$ 


Gall  bladder,  184 
Ganglion,  accessory,  362 

cervical,  365 

ciliary,  364 

Froriep's,  101,  124,  134,  35S 

geniculate,  100,  124,  360 

jugular,  101,  124,  362 

nodosum,  101,  124,  362 

otic,  365 

petrosal,  101,  124,  362 

prevertebral,  365 

semilunar,  100,  123,  359 

sphenopalatine,  365 

spinal,  123,  313 

spiral,  357 

submaxillar}-,  365 

superior,  101,  124,  362 

sympathetic,  364 

vestibular,  356 

visceral,  365 
Ganglion  cells,  315 

crest,  100,  313 
Gartner's  canal,  226,  237 
Gastrula,  38 
Gastrulation,  38 

in  mammals,  41 
Geniculate  bodies,  328 
Genital  ducts,  216,  218 

folds,  105,  128,  152,  205,  217 

glands,  105,  152,  216 

swelling,  232 

tubercle,  232 
Genitalia,  external,  232 
Germ  cells,  17 

layers,  12,  13,  37,  64 
derivatives  of,  64 
Germinal  area,  36 
Glands,  bulbo-urethral,  235 

cardiac,  179 

carotid,  366 

Fbner's,  161 

gastric,  179 

lachrymal,  381 

lingual,  162 

mammary.  307 

of  pregnancy,  244 

parotid,  161 

prostate,  235 

salivary,  161 


Glands,  sebaceous,  306 

sublingual,  161 

submaxillary,  161 

sudoriparous  (sweat),  307 

suprarenal,  152,  367 

vestibular,  235 
Glans  clitoris,  233 

penis,  234 
Glomerulus,  141,  205 
Glottis,  124,  160 
Graafian  follicle,  20,  224 
Gray  column  of  spinal  cord,  322 
Groove,  laryngotracheal,  173 

neural,  309 

primitive,  41 

urethral,  233 
Growth,  law  of,  13 
Gubernaculum  testis,  230 
Gyrus  dentatus,  344 

hippocampus,  344 


H-Emolymph  glands,  288 
Hair,  305 
Head-fold,  45 
Head-process,  44 
Heart,  chick,  52,  56 
human,  92,  255,  256 
pig,  105,  114,  128,  137,  152 
Hemispheres,  cerebellar,  334 

cerebral,  327,  346 
Henle's  loop,  211 
Hensen's  node  (knot),  44,  61 
Hepatic  diverticulum,  104,  116,  142 
Heredity,  theory  of,  31 
Hermaphroditism,  237 
Hernia,  diaphragmatic,  202 
inguinal,  232 
umbilical,  79 
Hind-brain,  chick,  54 

human,  325 
Hind-gut,  67,  91,  168 
Histogenesis,  defined,  13 

of  ectodermal  derivatives,  303 
of  entodermal  derivatives,  290 
of  mesodermal  derivatives,  291 
Human  embryos,  So 
estimated  age,  95 
of  Coste,  86 
of  Dandy,  85 
of  Eternod,  86 
of  His,  2.6  mm.,  87 
4.2  mm.,  88 
Normentafel,  94,  95 
of  Kollmann,  319 


394 


INDEX 


Human  embryos  of  Mall,  85,  98 

of  Peters,  85 

of  Spee,  85 

of  Thompson,  85 
Hydramnios,  83 
Hymen,  228 
Hyoid  arch,  98 
Hyomandibular  cleft,  98 
Hypophysis,  67,  89,  328,  337 
Hypospadias,  234 
Hypothalamus,  328,  330,  337 


Ileocecal  valve,  182 
Implantation  of  ovum,  239 
Incisive  fossa,  157 
Incus,  387 

Infundibulum,  328,  337 
Inguinal  canal,  229 

fold,  227,  229 
Inner  cell  mass,  36,  80 

epithelial  mass,  217 
Insula,  347 

Intermediate  cell  mass,  50,  62 
Internal  capsule,  343 

ear,  55 
Interstitial  cells  of  testis,  221 
Intestinal  glands,  182 

loop,  180 
Intestine,  human,  89,  180 

pig,  104,  128 
Introduction,  11 
Iris,  muscles  of,  380 
Isthmus,  327,  328 


Jacobson's  organ,  355, 372 
Joints,  298 


Kidney,  human,  207 
Knot,  primitive,  44,  61 


Labia  majora,  233.  237 

minora,  233,  234 
Lachrymal  gland,  381 

groove,  98 
Lamellated  corpuscle,  368 
Lamina  terminalis,  327,  340 
Laryngotracheal  groove,  173 
Larynx,  173,  174 
Layer,  chorioid,  of  eye,  380 

compact,  244 

ependymal,  133,  330 


Layer,  ganglion  cell,  379 
mantle,  133,  310,  321 
marginal,  133,  310,  323 
nerve-fiber,  379 
sclerotic,  of  eye,  380 
spongy,  244 
Lecithin,  17 
Lemniscus,  333,  336 
Lens  of  eye,  chick,  55 
human,  374 
capsule  of,  377 
fibers  of,  376 

pupillary  membrane  of,  378 
suspensory  ligament  of,  378 
vascular  capsule  of,  375 

Pig,  97 
Lesser  peritoneal  sac,  141,  197 
inferior  recess  of,  198 
superior  recess  of,  198 
Leucocytes,  253 
Ligament,  broad,  228 

coronary,  201 

duodeno-hepatic,  200 

falciform,  200 

gastro-hepatic,  200 

gastro-lienic,  200 

labial,  229 

lieno-renal,  200 

round,  227,  230 

triangular,  201 

vesico-umbilical,  216 
Ligamentum  labiale,  229 

ovarii,  229,  230 

scroti,  230 

teres,  286 

testis,  229 

venosum,  278 
Limbus  ovalis,  263 
Lip,  anlage  of,  154 

hare,  154 
Liver,  anlage  of,  67,  183 

bile  capillaries,  185 

chick,  67 

human,  183 

lobules  of,  186 

pig,  104,  127,  139 

quadrate  lobe,  201 

weight  of,  185 
Lizard,  germ  layers  of,  39,  40 
Lobes  of  cerebrum,  346 

of  liver,  201 
Lumbo-sacral  plexus,  354 
Lungs,  human,  89,  175 
apical  bud,  175 
changes  at  birth,  177 


INDEX 


395 


Lungs,  human,  stem  bud,  175 

pig,  126,  141 
Lunula,  308 
Lutein  cells,  224 
Lymph  glands,  287 

sacs,  286 
Lymphocytes,  253 


Macula  acustica,  384 

Malleus,  387 
Mammary  glands,  .^07 
Mammillary  body,  328,  339 

recess,  328 
Mandibular  arch,  98 

process,  87,  89,  98 
Mantle  layer,  133 
Marginal  layer.  133 
Margo  thalamicus,  325 
Marrow  of  bone,  251 
Massa  intermedia,  339 
Maturation,  24 

of  mouse  ovum,  28 
Maxillary  process,  87,  89,  97 
Maxillo-turbinal  anlage,  373 
Meatus,  external  auditory,  98,  120,  155,  388 
Meckel's  cartilage,  295 

diverticulum,  88 
Median  thyreoid,  102 
Mediastinum,  176 
Medulla  oblongata,  329 

of  kidney,  209 
Medullary  cords,  225 

velum,  335 
Membrana  tectoria,  385 
Membrane,  anal,  169,  213 

cloacal,  105,  169,  213 

pericardial,  195 

pharyngeal,  53,  168 

pleuro-pericardial,  192 

pleuro-peritoneal,  192 

tympanic,  388 

urogenital,  169,  213 
Mendel's  law,  31 
Menstruation,  20,  238 
Mesamoeboid.  251 
Mesencephalon,  chick,  54 

human,  89,  325,  328,  335 

pig,  100 
Mesenchyma,  chick,  51,  63 

human,  291 
Mesentery,  104,  141,  188,  190,  201 

dorsal,  80,  iqo.  201 
Mesocardium,  52,  188,  256 
Mesocolon,  iqo,  201,  202 


Mesoderm,  amphibian,  43 
Amphiozu 

bird,  43 

mammal,  45.  4'» 

somatic,  50,  63,  291 

splanchnic,  50,  63,  291 
Mesodermal  segments,  45,  50,  61 
Me ^ xluodenum,  190,  201 
Mesogastrium,  190 
Mesonephric  duct,  92,  105 

fold,  205,  206 

pig,  105,  128,  141,  144 

tubules,  205 
Mesonephros,  62,  92,  128,  152,  205 
Mesorchium,  217 
Mesorectum,  190,  202 
Mesothelium,  64,  291 
Mesovarium,  217 
Metamerism,  12 
Metanephros,  105,  120,  128,  144,  152,  207,  208 

calyces  of,  207 

collecting  tubules,  128,  207 

cortex,  209 

medulla,  209 

pelvis,  128,  207 

tubules,  208 

ureter,  128,  207 
Metathalamus,  328 
Metencephalon,  S9,  100,  329,  333 
Methods  of  dissection,  145 

of  study,  14 
Mid-brain,  54,  325,  328,  335 
Mid-gut,  67,  168 
Milk  line,  307 
Mitochondria  sheath,  22 
Mitosis,  22 

significance  of,  31 
Modiolus,  386 
Monaster,  23 
Monkey,  ovum  of,  20 
Mons  veneris,  233,  237 
Morula,  t,^ 

Mouse,  ovum  of,  28,  30 
Mucous  tissue,  79 
Muellerian  ducts,  218 

tubercle,  227 
Muscle,  cardiac,  299 

histogenesis  of,  298 

plate,  119 

smooth,  298 

striated  voluntary,  299 
Muscles,  morphogenesis  of,  301 

of  diaphragm.  195 
Myelencephalon,  89,  100,  112,  320,  330 
Myelocytes,  253 


396 


INDEX 


373 
See  Cervical  flexure 


Myocardium,  52,  106 
Myotome,  chick,  62 

human,  299 

Pig,  119 


Nail,  human,  308 

Naris,  370 

Nasal  passages,  157 

pits,  human,  370 
pig,  97,  120 

processes,  153,  370 
Naso-turbinal  anlage, 
Neck-bend,  147,  346. 
Nephrogenic  cord,  205,  208 

tissue,  128,  210 
Nephrostome,  203 
Nephrotome,  chick,  62 

human,  204 
Nerve  fibers,  histogenesis  of,  310 
Nerves,  abducens,  123,  359 

acoustic,  100,  124,  356 

cerebral,  355 

chorda  tympani,  124,  361 

facial,  100,  123,  360 

femoral,  354 

glossopharyngeal,  101,  124,  361 

hypoglossal,  101,  124,  358 

mandibular,  123,  359 

maxillary,  123,  359 

oculomotor,  100,  123,  358 

olfactory,  123,  355 

ophthalmic,  123,  359 

optic,  123,  356 

phrenic,  192,  353 

sciatic,  354 

spinal,  124,  351 

accessory,  101,  124,  362 

superficial  petrosal,  361 

sympathetic,  364 

trigeminal,  100,  123,  359 

trochlear,  123,  359 

vagus,  101,  124,  362 
Nervous  system,  54,  99,  122 
Neural  crest,  100,  313 

folds,  45,  309 

groove,  45,  309 

tube,  45,  330 
Neurenteric  canal,  41 
Neuroblasts,  311,  313 
Neuroglia,  origin  of,  310,  317 
Neuromeres,  54,  112 
Neurone  theory,  315 
Neurones,  312,  313 
Neuropore,  anterior,  54,  319 


Neutrophiles,  254 
Node  of  Ranvier,  316 

primitive,  44,  61 
Nodulus  cerebelli,  334 
Normoblasts,  252 
Notochord,  origin  of,  42,  46 

chick,  49 

human,  46 
Notochordal  canal,  41 

plate,  40 
Nuclei  of  origin,  332 

terminal,  332 
Nucleolus,  17 
Nucleus  ambiguus,  362 

caudatus,  343 

cuneatus,  332 

dentatus,  335 

gracilis,  332 

lenticular,  343 

of  ovum,  17 

of  pons,  333 

olivary,  333 

ruber,  335 


Obex,  332 

(Esophagus,  151,  177.     See  Esophagus 

Olfactory  bulb  (lobe),  344 

fossa,  369 

nerves,  123,  335 

organ,  369 

placode,  369 

tracts,  344 
Olivary  body,  inferior,  333 
Omental  bursa,  197 

inferior  recess  of,  198 
Omentum,  141,  178,  199 
Oocyte,  28 
Oogonia,  28 
Operculum,  347 
Optic  chiasma,  328 

cup,  67,  374 

nerve,  123,  356 

recess,  341 

stalk,  374 

vesicle,  49,  54,  328 
Ora  serrata,  379 
Organ  of  Jacobson,  355,  372 

spiral,  384 
Ostium  abdominale,  218 

vaginae,  228 
Otic  vesicle,  55,  382 
Otoconia,  384 

Otocyst,  55,  67,  89,  100,  133,  382 
Ova,  primordial,  223 


i.\i)i;\ 


397 


Ovary,  221 

compared  with  testis,  225 
descent  of,  230 

Ovulation,  20,  239 
Ovum,  human,  17 
implantation  of,  239 
maturation  of,  28 
structure  of,  amphibian,  17 
bird,  17 
frog,  35 
monkey,  21 
rabbit,  36 


Palate,  cleft,  158 

hard,  155,  157 

premaxillarv,  371 
Pallium  of  cerebrum,  327 
Pancreas,  human,  186 

accessory  duct  of,  1S7 
islands  of,  188 

pig,  104,  117,  127,  142 
Papilla;  of  tongue,  160 

renal,  209 

vallate,  161 
Paradidymis,  226,  237 
Parathyrcoid  gland,  90,  172 
Parolfactory  area,  344 
Paroophoron,  237 
Pars  optica  hypothalamica,  337 
Penis,  152,  237 
Perforated  space,  344 
Perforatorium,  21 
Pericardial  cavity,  chick,  50 

human,  188,  190 
Perichondrium,  294 
Periosteum,  297 
Peritoneum,  141,  291 
Phallus,  152,  232,  237 
Pharyngeal  membrane,  53,  S9 

pouches,  90,  103,  125,  134,  169 
Tharynx,  human,  89,  169 

pig,  124 
Pia  mater,  133 

Pig  embryos,  41,  97,  120,  146,  148,  151 
dissection  of,  145 
sections  of  6  mm.,  in 
10  mm.,  132 
Pigment  layer  of  retina,  379 
Pillars  of  Corti,  384 
Pineal  gland,  328 
Pituitary  body,  337 
Placenta,  human,  82,  245 
cotyledons  of,  249 
intervillous  spaces  of,  249 


Placenta,  human,  position  of,  250 

relation  of  fetus  to,  249 

vessels  of,  249 

pig,  78 
Placodes,  auditory,  55,  382 

olfactory,  369 

optic,  55,  374 
Plate,  alar,  321,  330 

basal,  321,  330 

closing,  169 

cutis,  119 

floor,  321,  330 

muscle,  119 

neural,  309 

notochordal,  40 

roof,  321,  330 

urethral,  232 
Pleura,  177 
Pleural  cavity,  177 
Pleuro-pericardial  cavity,  50,  190 
Pleuro-pcritoneal  cavity,  5c,  188,  190 
Plexus,  brachial,  353 

cardiac,  365 

chorioid,  150,  332,  336,  340 

cceliac,  365 

lumbo-sacral,  354 
Plica  venae  cavae,  142,  201,  282 
Polar  bodies,  28 
Polocytes,  28 
Polydactylism,  84 
Polyspermy,  30 
Pons,  147,  329 
Pontine  flexure,  147,  326 
Preformation  theory,  n 
Pregnancy,  abdominal,  30 

tubular,  30 
Premyelocytes,  252 
Prepuce  of  clitoris,  233 

of  penis,  234 
Primary  excretory  ducts,  62,  204 
Primates,  77 
Primitive  choanal,  371 

groove,  41,  61 

knot,  44,  61 

node,  44,  61 

streak,  40,  61 
Proamniotic  area,  44,  48 
Process,  lateral   nasal,    153,  370 

mandibular,  S7,  89,  98,  370 

maxillary,  87,  89,  97,  370 

median  frontal,  370 
nasal,  153,370 

palatine,  155,  373 
Pronephros,  203 
Pronuclei,  union  of,  30 


/ 


393 


INDEX 


Pronucleus,  28,  29 
Prosencephalon,  54,  325 
Prostate  gland,  235 
Pyramidal  cells,  349 
Pyramids,  2>2>2> 
of  kidney,  209 


Quadrate  lobe  of  liver,  201 


Ramus  communicans,  352,  364 
Rathke's  pocket,  67,  169 
Receptaculum  chyli,  287 
Recess,  lateral,  331 
Rectum,  127,  143,  152 
Reduction  of  chromosomes,  27 
Reference,  titles  for,  15 
Regression,  14 
Renal  anomalies,  213 

arteries,  213 

columns,  209 

corpuscles,  206,  207 

cortex,  209 

papillae,  209 

pelvis,  207 

pyramids,  209 

tubules,  208 
Respirator}-  epithelium,  291 
Rete  ovarii,  221,  225 

testis,  220 
Reticular  formation,  332 

tissue,  292 
Retina,  378 

Rhinencephalon,  147,  327,  339,  344 
Rhombencephalon,  54,  89,  325 
Rhombic  lip,  332 
Rhomboidal  sinus,  49,  54,  67 
Ridge,  pulmonary,  193 
Roots  of  spinal  nerves,  139 


Saccules,  383 
Saccus  vaginalis,  231 
Scake,  386 
Sclerotome,  291 
Scrotum,  234 

ligament  of,  230 
Sections,  chick,  fifty-hours,  69 
thirty-six-hours,  57 
twenty-five-hours,  50 
pig,  6  mm.,  in 
10  mm.,  132 
Seessel's  pocket,  89,  101,  169 
Segmental  zone,  73 


Segmentation  of  ovum,  33 
Amphioxus,  33 
frog,  34 
mammals,  36 
reptiles  and  birds,  35 
Segments,  primitive,  119.     See   Mesodermal  seg- 
ments 
Seminal  vesicle,  226 
Sense  organs,  chick,  54,  66 
human,  368 
pig,  122 
Septum,  dorsal  median,  321 

interatrial,  259 

interventricular,  264 

membranaceum,  267 

nasal,  372 

pellucidum,  344 

primum,  106,  260 

secundum,  260 

spurium,  128,  262 

transversum,  91,  114,  140,  152,  188,  191 
Sex,  determination  of,  31 
Sheath,  medullary,  316 

mitochondria,  22 

myelin,  316 

neurilemma,  316 
Sinus, cavernous,  133,  279 

cervical,  97 

coronary,  279 

frontal,  373 

lateral,  279 

maxillary,  373 

petrosal,  279 

rhomboidal,  49,  55 

sphenoidal,  373 

superior  sagittal,  279 

urogenital,  85,  127,  142,  213,  218,  234 

venosus,  67,  137,  257,  261 
Sinusoids  of  liver,  67,  184,  276 
Somatopleure,  42,  63 
Somites.     See  Segments 
Spermatic  cord,  232 
Spermatid,  26 
Spermatocyte,  24 
Spermatogenesis,  24 
Spermatogonia,  24,  221 
Spermatozoon,  21 
Spinal  cord,  320 
Spireme,  23 

Splanchnopleure,  42,  63 
Spleen,  200,  288 
Spongioblasts,  311 
Stapes,  387 
Stoerck's  loop,  212 
Stomodseum,  67 


INDEX 


399 


Stomach,  89,  103,  115,  127,  141,  177 
Stratum  corneum,  305 

germinativum,  304 

granulosum,  224,  305 

lucidum,  305 
Streak,  primitive,  40 
Stroma  of  ovary,  223 
Sulci  of  cerebrum,  349 
Sulcus  centrale,  348 

coronary,  232 

hypotnalamicus,  328,  337 

limitans,  321,  330,  337 

spirale,  384 

terminalis,  158 
Suprarenal  glands,  152 
Sweat  glands,  307 
Sympathetic  system,  364 

Tactile  corpuscles,  368 
Taenia,  332 
Tail-fold,  66 
Tail-gut,  104 
Tarsius,  39 
Taste-buds,  368 
Teeth,  anlages  of,  162 

cement  of,  165 

dental  lamina  of,  162 
papilla,  162 
pulp,  165 
sac,  165 

dentine,  165 

enamel,  163 

odontoblasts,  165 
Tegmentum,  328 
Tela  chorioidea,  327 
Telencephalon,  chick,  66 

human,  89,  339 

pig,  100 
Telolecithal  ova,  33 
Tendon,  293 
Terminal  nuclei,  332 
Testis,  218 

anomalies  of,  221 

cords,  220 

descent  of,  230 

gubernaculum  of,  230,  231 

intermediate  cords  of,  220 

interstitial  cells  of,  221 

mediastinum,  220 

rete,  220 

tubuli  contorti,  221 
recti,  221 
septula,  221 
Tetrads,  27 

Thalamus,  150,  328,  343 
Theca  folliculi.  224 


Thomes'  fibers,  165 
Thj  mic  corpuscles,  172 
Thymus,  171 
Thy  Hyoglossal  duct,  172 
Thyreoid  gland,  172 

human,  91,  169,  172 
pig,  102,  126 
Tissue,  adipose,  294 

bone,  295 

cartilage,  294 

1  lassification  of,  64 

connective,  292 

differentiation  of,  13 

elastic,  293 

muscle,  298 

nervous,  309 

origin  of,  64 

reticular,  292 

supporting,  292 

white  fibrous,  292 
Titles  for  reference,  15 
Tongue  of  pig,  124,  151,  158 
Tonsil,  human,  159 

origin  of,  171 

palatine,  90,  170 
Trabecular  of  liver,  184 
Trachea,  human,  173,  174 

pig,  103,  126,  151 
Tractus  solitarius,  362 
Triangular  ligament  of  liver,  201 
Trigeminal  nerve,  100,  123,  359 
Trophectoderm,  36,  80,  239 
Trophoderm,  81,  239,  243 
Tuber  cinereum,  328,  339 
Tuberculum  impar,  91,  102,  124,  159 
Tubules  of  kidney,  211 
Tubuli  contorti,  221 

recti,  22 
Tunica  albuginea,  220,  223 

vaginalis,  231 

vasculosa  lentis,  375 
Turbinal  anlages,  151 
Tympanum,  386 

Ultimobraxchial  body,  172 
Umbilical  cord,  human,  79 

of  pig,  79 
Umbilicus,  79,  216 
Unguiculates,  77 
Ungulates,  36,  77,  82 
Urachus,  85,  216 
Ureter,  152,  207,  215 
Urethra,  152,  213,  215,  234,  237 
Urethral  groove,  233 
Uriniferous  tubules,  211 
Urogenital  ducts,  153 


4oo 


INDEX 


Urogeital  folds,  216 

"organs,  92,  152,  203 

sinus,  85,  127,  142,  213,  218,  234 

system,  127,  203 
Uterine  tube,  226 
Uterus,  226,  227,  238 

anomalies  of,  228 

fundus  of,  228 

growth  of,  228 

ligaments  of,  228,  229 

masculinus,  227 

menstrual  changes  of,  238 
Utriculus,  384 
Uvula,  158 

Vagina,  226,  227 

fornices  of,  228 

masculina,  226   237 
Valves,  bicuspid,  128,  267 

Eustachian,  262 

ileo-csecal,  182 

of  coronary  sinus,  263 

of  sinus  venosus,  128,  258,  261,  262 

semilunar,  266 

Thebesian,  263 

tricuspid,  128,  267 
Veins,  allantoic,  of  chick,  76,  77 

anonymous,  171,  279 

anterior  cardinal,  68,  93,  109,  130,  268,  279 

axillary,  283 

azygos,  283 

basilic,  283 

brachial,  283 

cephalic,  283 

cerebral,  279 

changes  at  birth,  286 

common  cardinal,  68,  93,  109,  269,  279 

femoral,  284 

gluteal,  284 

hepatic,  278,  282 

iliac,  281,  283 

ischiadic,  284 

jugular,  130,  279 

linguo-facial,  109 

lumbar,  283 

mesenteric,  superior,  278 

of  extremities,  283 

of  head,  279 

of  pig,  106,  107,  130 

ophthalmic,  279 

ovarian,  283 

portal,  107,  132,  276 

posterior  cardinal,  68,  93,  109,  130,  269,  280 

pulmonary,  177,  264 

renal,  283 

subcardinal,  109,  130,  281 


Veins,  subclavian,  283 
supracardinal,  283 
suprarenal,  283 
umbilical,  human,  93,  276 
pig,  107,  130 

vitelline,  49,  93,  107,  132,  268,  276 

vitello-umbilical,  93,  268 
Velum,  medullary,  335 
Vena  capitis  lateralis,  279 
medialis,  279 

cava  inferior,  in,  130,  282,  283 
superior,  279 

porta,  276,  278 
Venous  system,  276 
Ventral  roots,  313 
Ventricle,  fifth,  344 

first,  328 

fourth,  329 

lateral,  327 

of  heart,  67,  259,  264 

terminal,  324 

third,  328 
Vermiform  process,  182 
Vermis  cerebelli,  334 
Vernix  caseosa,  83 
Vertebrae,  human,  297 

pig,  119,  149 
centra  of,  151 
Vesicle,  brain,  320 

branchial,  170 

cerebral,  100 

lens,  97 

optic,  49,  54,  328 

seminal,  226 
Vesico-urethral  anlage,  213 
Vestibular  glands,  235 
Vestibule,  383 
Villi,  origin  of,  82,  240 

of  chorion,  80,  240,  246 

of  intestine,  182 

vessels  of,  240 
Vitelline  membrane,  48 
Vitreous  body  of  eye,  377 
Vocal  cords,  174 

Whole  embryos  for  study,  146 

Yolk,  17 

Yolk-cavity,  37 

Yolk-sac,  75,  77,  85,  86 

Yolk-stalk,  75,  79,  86,  87,  88,  168,  180 

Zona  pellucida,  17 
Zone,  segmental,  73 
Zonula  ciliata  (of  Zinn),  378 


■ 


